diff --git a/package-lock.json b/package-lock.json
index baa120a..f0c6ebd 100644
--- a/package-lock.json
+++ b/package-lock.json
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+      "license": "MIT",
+      "funding": {
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+      }
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+      "resolved": "https://registry.npmjs.org/@vitest/utils/-/utils-4.1.10.tgz",
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+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
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@@ -3634,6 +3766,16 @@
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+      "dev": true,
+      "license": "MIT",
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@@ -3808,6 +3950,16 @@
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+      "license": "MIT",
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@@ -4267,6 +4419,13 @@
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@@ -4367,6 +4526,16 @@
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@@ -4382,6 +4551,16 @@
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+      "dev": true,
+      "license": "Apache-2.0",
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@@ -5089,6 +5268,16 @@
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+      "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-0.30.21.tgz",
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+      "license": "MIT",
+      "dependencies": {
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@@ -5285,6 +5474,20 @@
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+      "resolved": "https://registry.npmjs.org/obug/-/obug-2.1.3.tgz",
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+      "funding": [
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+        "https://opencollective.com/debug"
+      ],
+      "license": "MIT",
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@@ -5314,6 +5517,13 @@
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@@ -6296,6 +6513,20 @@
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@@ -6510,6 +6758,16 @@
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@@ -6803,6 +7061,109 @@
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     "node_modules/void-elements": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/void-elements/-/void-elements-3.1.0.tgz",
@@ -6818,6 +7179,23 @@
       "integrity": "sha512-dpojBhNsCNN7T82Tm7k26A6G9ML3NkhDsnw9n/eoxSRlVBB4CEtIQ/KTCLI2Fwf3ataSXRhYFkQi3SlnFwPvPQ==",
       "license": "MIT"
     },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+      "integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/wrap-ansi": {
       "version": "9.0.2",
       "resolved": "https://registry.npmjs.org/wrap-ansi/-/wrap-ansi-9.0.2.tgz",
diff --git a/package.json b/package.json
index 7d9fc8a..9e53f98 100644
--- a/package.json
+++ b/package.json
@@ -13,6 +13,8 @@
     "format": "biome format --write .",
     "format:check": "biome format .",
     "check": "biome check .",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "type-check": "tsc --noEmit",
     "preview": "vite preview",
     "prepare": "husky"
@@ -87,7 +89,8 @@
     "postcss": "^8.4.31",
     "tailwindcss": "^3.4.19",
     "typescript": "^5.2.2",
-    "vite": "^7.3.1"
+    "vite": "^7.3.1",
+    "vitest": "^4.1.10"
   },
   "lint-staged": {
     "*.{ts,tsx,json}": [
diff --git a/src/utils/formatTraffic.test.ts b/src/utils/formatTraffic.test.ts
new file mode 100644
index 0000000..659a69d
--- /dev/null
+++ b/src/utils/formatTraffic.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it, vi } from 'vitest';
+import { formatTraffic } from './formatTraffic';
+
+vi.mock('../i18n', () => ({
+  default: { t: (_key: string, fallback: string) => fallback },
+}));
+
+describe('formatTraffic', () => {
+  it('formats terabytes from 1000 GB up', () => {
+    expect(formatTraffic(1000)).toBe('1.0 TB');
+    expect(formatTraffic(2500)).toBe('2.5 TB');
+  });
+
+  it('formats gigabytes from 1 GB up', () => {
+    expect(formatTraffic(1)).toBe('1.0 GB');
+    expect(formatTraffic(999.94)).toBe('999.9 GB');
+  });
+
+  it('formats sub-gigabyte values as megabytes', () => {
+    expect(formatTraffic(0.5)).toBe('512 MB');
+    expect(formatTraffic(0)).toBe('0 MB');
+  });
+});
diff --git a/src/utils/period.test.ts b/src/utils/period.test.ts
new file mode 100644
index 0000000..fbb0692
--- /dev/null
+++ b/src/utils/period.test.ts
@@ -0,0 +1,29 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { getMonthToDateRange, isMonthToDate } from './period';
+
+describe('period', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('builds a month-to-date range with zero-padded local dates', () => {
+    vi.setSystemTime(new Date(2026, 2, 5));
+    expect(getMonthToDateRange()).toEqual({ startDate: '2026-03-01', endDate: '2026-03-05' });
+  });
+
+  it('recognises the current month-to-date period', () => {
+    vi.setSystemTime(new Date(2026, 6, 13));
+    expect(isMonthToDate({ startDate: '2026-07-01', endDate: '2026-07-13' })).toBe(true);
+  });
+
+  it('rejects day-based and mismatched periods', () => {
+    vi.setSystemTime(new Date(2026, 6, 13));
+    expect(isMonthToDate({ days: 30 })).toBe(false);
+    expect(isMonthToDate({ startDate: '2026-07-01', endDate: '2026-07-12' })).toBe(false);
+    expect(isMonthToDate({})).toBe(false);
+  });
+});
diff --git a/src/utils/safeRedirect.test.ts b/src/utils/safeRedirect.test.ts
new file mode 100644
index 0000000..e368483
--- /dev/null
+++ b/src/utils/safeRedirect.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from 'vitest';
+import { getSafeRedirectPath } from './safeRedirect';
+
+describe('getSafeRedirectPath', () => {
+  it('returns / for empty values', () => {
+    expect(getSafeRedirectPath(null)).toBe('/');
+    expect(getSafeRedirectPath(undefined)).toBe('/');
+    expect(getSafeRedirectPath('')).toBe('/');
+  });
+
+  it('keeps plain absolute in-app paths', () => {
+    expect(getSafeRedirectPath('/')).toBe('/');
+    expect(getSafeRedirectPath('/dashboard')).toBe('/dashboard');
+    expect(getSafeRedirectPath('/sub/page?tab=devices&x=1')).toBe('/sub/page?tab=devices&x=1');
+  });
+
+  it('rejects protocol-relative and absolute URLs', () => {
+    expect(getSafeRedirectPath('//evil.com')).toBe('/');
+    expect(getSafeRedirectPath('https://evil.com/path')).toBe('/');
+    expect(getSafeRedirectPath('http://evil.com')).toBe('/');
+  });
+
+  it('rejects non-path schemes', () => {
+    expect(getSafeRedirectPath('javascript:alert(1)')).toBe('/');
+    expect(getSafeRedirectPath('data:text/html,x')).toBe('/');
+  });
+
+  it('rejects URL-encoded host smuggling', () => {
+    expect(getSafeRedirectPath('%2F%2Fevil.com')).toBe('/');
+    expect(getSafeRedirectPath('/redirect%3A%2F%2Fevil.com')).toBe('/');
+  });
+
+  it('rejects malformed percent-encoding instead of throwing', () => {
+    expect(getSafeRedirectPath('/%E0%A4%A')).toBe('/');
+  });
+});
diff --git a/src/utils/sanitizeSurrogates.test.ts b/src/utils/sanitizeSurrogates.test.ts
new file mode 100644
index 0000000..cebbd78
--- /dev/null
+++ b/src/utils/sanitizeSurrogates.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from 'vitest';
+import { sanitizeSurrogates } from './sanitizeSurrogates';
+
+describe('sanitizeSurrogates', () => {
+  it('returns surrogate-free strings unchanged', () => {
+    expect(sanitizeSurrogates('')).toBe('');
+    expect(sanitizeSurrogates('https://host/sub/uuid#remark')).toBe('https://host/sub/uuid#remark');
+  });
+
+  it('keeps valid surrogate pairs intact', () => {
+    expect(sanitizeSurrogates('server 😀 name')).toBe('server 😀 name');
+  });
+
+  it('replaces a lone high surrogate', () => {
+    expect(sanitizeSurrogates('abc\uD83D')).toBe('abc�');
+    expect(sanitizeSurrogates('\uD83Dxyz')).toBe('�xyz');
+  });
+
+  it('replaces a lone low surrogate', () => {
+    expect(sanitizeSurrogates('abc\uDE00def')).toBe('abc�def');
+  });
+
+  it('makes a truncated-emoji string safe for encodeURI', () => {
+    const broken = 'remark \uD83D truncated';
+    expect(() => encodeURI(broken)).toThrow();
+    expect(() => encodeURI(sanitizeSurrogates(broken))).not.toThrow();
+  });
+});
diff --git a/src/utils/validation.test.ts b/src/utils/validation.test.ts
new file mode 100644
index 0000000..501418b
--- /dev/null
+++ b/src/utils/validation.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, it } from 'vitest';
+import { isValidEmail } from './validation';
+
+describe('isValidEmail', () => {
+  it('accepts common addresses', () => {
+    expect(isValidEmail('user@example.com')).toBe(true);
+    expect(isValidEmail('a.b+tag@sub.domain.io')).toBe(true);
+  });
+
+  it('rejects malformed addresses', () => {
+    expect(isValidEmail('')).toBe(false);
+    expect(isValidEmail('plain')).toBe(false);
+    expect(isValidEmail('user@host')).toBe(false);
+    expect(isValidEmail('user @example.com')).toBe(false);
+    expect(isValidEmail('user@exa mple.com')).toBe(false);
+  });
+});
diff --git a/tsconfig.node.json b/tsconfig.node.json
index ec78806..b6af3e1 100644
--- a/tsconfig.node.json
+++ b/tsconfig.node.json
@@ -8,5 +8,5 @@
     "resolveJsonModule": true,
     "types": ["node"]
   },
-  "include": ["vite.config.ts"]
+  "include": ["vite.config.ts", "vitest.config.ts"]
 }
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..fc838d3
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+
+// Kept separate from vite.config.ts so the app build config stays untouched;
+// utility tests run in a plain node environment (no jsdom needed).
+export default defineConfig({
+  test: {
+    include: ['src/**/*.test.{ts,tsx}'],
+    environment: 'node',
+  },
+});