fix: resolve all 14 ESLint warnings across the codebase

- Add varsIgnorePattern to no-unused-vars for destructuring patterns
- Fix all react-hooks/exhaustive-deps by adding missing dependencies
- Refactor useAnimatedNumber to use ref instead of stale state closure
- Wrap handleLinkResult in useCallback for stable deps
- Extract OAuth utilities from OAuthCallback.tsx to utils/oauth.ts
- Extract background config utilities to utils/backgroundConfig.ts
- Remove unused catch parameter in GiftSubscription

src/utils/backgroundConfig.ts

@@ -0,0 +1,63 @@
+import type { AnimationConfig, BackgroundType } from '@/components/ui/backgrounds/types';
+
+const ANIMATION_CACHE_KEY = 'cabinet_animation_config';
+const MAX_CONFIG_JSON_LENGTH = 10_000;
+
+const VALID_TYPES: ReadonlySet<string> = new Set<BackgroundType>([
+  'aurora',
+  'sparkles',
+  'vortex',
+  'shooting-stars',
+  'background-beams',
+  'background-beams-collision',
+  'gradient-animation',
+  'wavy',
+  'background-lines',
+  'boxes',
+  'meteors',
+  'grid',
+  'dots',
+  'spotlight',
+  'ripple',
+  'none',
+]);
+
+export function validateConfig(parsed: unknown): AnimationConfig | null {
+  if (typeof parsed !== 'object' || parsed === null || Array.isArray(parsed)) return null;
+  const obj = parsed as Record<string, unknown>;
+  if (typeof obj.enabled !== 'boolean') return null;
+  if (typeof obj.type !== 'string' || !VALID_TYPES.has(obj.type)) return null;
+  if (typeof obj.opacity !== 'number') return null;
+  if (typeof obj.blur !== 'number') return null;
+  if (obj.settings != null && (typeof obj.settings !== 'object' || Array.isArray(obj.settings)))
+    return null;
+  return {
+    enabled: obj.enabled,
+    type: obj.type as BackgroundType,
+    opacity: Math.max(0, Math.min(1, obj.opacity)),
+    blur: Math.max(0, Math.min(50, obj.blur)),
+    settings: (obj.settings as Record<string, unknown>) ?? {},
+    reducedOnMobile: typeof obj.reducedOnMobile === 'boolean' ? obj.reducedOnMobile : true,
+  };
+}
+
+export function getCachedConfig(): AnimationConfig | null {
+  try {
+    const cached = localStorage.getItem(ANIMATION_CACHE_KEY);
+    if (!cached || cached.length > MAX_CONFIG_JSON_LENGTH) return null;
+    return validateConfig(JSON.parse(cached));
+  } catch {
+    return null;
+  }
+}
+
+export function setCachedConfig(config: AnimationConfig) {
+  try {
+    localStorage.setItem(ANIMATION_CACHE_KEY, JSON.stringify(config));
+  } catch {}
+}
+
+export function setCachedAnimationConfig(config: AnimationConfig) {
+  const validated = validateConfig(config);
+  if (validated) setCachedConfig(validated);
+}