fix: allow clearing number inputs and add validation

- Add inputHelpers utility for number inputs that allow empty values
- Update tariff, campaign, promo offers, and server forms
- Add validation for required fields (deviceLimit, tierLevel, dailyPrice)
- Add placeholders and error indicators for empty required fields
- Add fieldRequired and dailyPriceRequired translations

src/utils/inputHelpers.ts

@@ -0,0 +1,48 @@
+import { ChangeEvent } from 'react';
+
+/**
+ * Creates an onChange handler for number inputs that allows empty values while typing.
+ * This fixes the issue where inputs can't be cleared because `parseInt('') || 0` immediately fills with 0.
+ *
+ * @param setter - The state setter function
+ * @param min - Optional minimum value constraint
+ * @param max - Optional maximum value constraint
+ */
+export function createNumberInputHandler(
+  setter: (value: number | '') => void,
+  min?: number,
+  max?: number,
+) {
+  return (e: ChangeEvent<HTMLInputElement>) => {
+    const val = e.target.value;
+
+    if (val === '') {
+      setter('');
+      return;
+    }
+
+    let num = parseFloat(val);
+
+    if (isNaN(num)) return;
+
+    if (min !== undefined) num = Math.max(min, num);
+    if (max !== undefined) num = Math.min(max, num);
+
+    setter(num);
+  };
+}
+
+/**
+ * Converts a number|'' state value to a number, using a default for empty.
+ */
+export function toNumber(value: number | '', defaultValue = 0): number {
+  return value === '' ? defaultValue : value;
+}
+
+/**
+ * Gets the display value for a kopeks field (converts to rubles).
+ * Returns '' if the value is empty, otherwise divides by 100.
+ */
+export function kopeksToDisplay(value: number | ''): number | '' {
+  return value === '' ? '' : value / 100;
+}