feat(plugins): add date formatting utility for i18n (Phase 4 partial)

- Create plugin-i18n-date.util.ts with formatDateForPlugin()
- Supports 5 predefined formats: short, medium, long, time, datetime
- Uses Intl.DateTimeFormat for locale-aware formatting
- Handles Date, string, and number inputs
- Graceful fallback to English for invalid locales

Part of Phase 4: Plugin API Extensions

src/app/plugins/plugin-i18n-date.util.ts

@@ -0,0 +1,103 @@
+/**
+ * Format dates for plugins using locale-aware Intl.DateTimeFormat
+ * Supports predefined formats and handles various date inputs
+ */
+
+type DateInput = Date | string | number;
+type DateFormat = 'short' | 'medium' | 'long' | 'time' | 'datetime';
+
+/**
+ * Format a date according to predefined format and locale
+ */
+export const formatDateForPlugin = (
+  dateInput: DateInput,
+  format: DateFormat,
+  locale: string,
+): string => {
+  // Parse input to Date object
+  const date = parseDate(dateInput);
+  if (!date) {
+    return '';
+  }
+
+  // Get format options
+  const options = getFormatOptions(format);
+  if (!options) {
+    return '';
+  }
+
+  // Format using Intl.DateTimeFormat
+  try {
+    return new Intl.DateTimeFormat(locale, options).format(date);
+  } catch (error) {
+    // Fallback to English if locale is invalid
+    return new Intl.DateTimeFormat('en', options).format(date);
+  }
+};
+
+/**
+ * Parse various date inputs into Date object
+ */
+const parseDate = (input: DateInput): Date | null => {
+  if (input instanceof Date) {
+    return isNaN(input.getTime()) ? null : input;
+  }
+
+  if (typeof input === 'number') {
+    const date = new Date(input);
+    return isNaN(date.getTime()) ? null : date;
+  }
+
+  if (typeof input === 'string') {
+    const date = new Date(input);
+    return isNaN(date.getTime()) ? null : date;
+  }
+
+  return null;
+};
+
+/**
+ * Get Intl.DateTimeFormat options for predefined formats
+ */
+const getFormatOptions = (format: DateFormat): Intl.DateTimeFormatOptions | null => {
+  switch (format) {
+    case 'short':
+      return {
+        year: '2-digit',
+        month: 'numeric',
+        day: 'numeric',
+      };
+
+    case 'medium':
+      return {
+        year: 'numeric',
+        month: 'short',
+        day: 'numeric',
+      };
+
+    case 'long':
+      return {
+        year: 'numeric',
+        month: 'long',
+        day: 'numeric',
+      };
+
+    case 'time':
+      return {
+        hour: 'numeric',
+        minute: '2-digit',
+      };
+
+    case 'datetime':
+      return {
+        year: '2-digit',
+        month: 'numeric',
+        day: 'numeric',
+        hour: 'numeric',
+        minute: '2-digit',
+      };
+
+    default:
+      return null;
+  }
+};