refactor(content): simplify content script structure

- Remove messageHandler.ts (6-line pass-through facade)
- Import contextMenuHandler directly from content.ts
- Replace local getI18nText() with shared getMessage() from utils/chromeI18n

Reduces content script files from 3 to 2 and eliminates i18n logic duplication.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entrypoints/content.ts

@@ -1,10 +1,10 @@
 import '../../.wxt/types/imports.d.ts';
-import { initMessageHandler } from './content/messageHandler';
+import { initContextMenuHandler } from './content/contextMenuHandler';
 
 export default defineContentScript({
   matches: ['<all_urls>'],
   runAt: 'document_end',
   main() {
-    initMessageHandler();
+    initContextMenuHandler();
   },
 });

src/entrypoints/content/contextMenuHandler.ts

@@ -1,17 +1,11 @@
 import type { ContextMenuClickedPayload } from '@/utils/messages';
 import { MessageAction, onMessage } from '@/utils/messages';
 import { getTextStats } from '@/utils/textStatistics';
+import { getMessage } from '@/utils/chromeI18n';
 import { hidePopover, showTextStatsResult, showTimestampResult } from './uiPopover';
 
-function getI18nText(key: string, fallback: string): string {
-  if (typeof chrome !== 'undefined' && chrome.i18n) {
-    return chrome.i18n.getMessage(key) || fallback;
-  }
-  return fallback;
-}
-
 function convertTimestamp(input: string): string {
-  const invalidText = getI18nText('invalidTimestamp', 'Invalid Timestamp');
+  const invalidText = getMessage('invalidTimestamp') || 'Invalid Timestamp';
   const num = Number(input.trim());
 
   if (isNaN(num)) {

src/entrypoints/content/messageHandler.ts

@@ -1,5 +0,0 @@
-import { initContextMenuHandler } from './contextMenuHandler';
-
-export function initMessageHandler(): void {
-  initContextMenuHandler();
-}