feat: add email templates with placeholder variables and composer integration

- Reusable email templates with local storage persistence
- Dynamic placeholder variables ({{recipientName}}, {{date}}, etc.) with auto-fill
- Template manager modal with category filtering and search
- Template picker integrated in composer toolbar (Ctrl+Shift+T)
- Settings tab for template management
- 48 unit tests for template utilities
- i18n support for all 8 languages

hooks/use-keyboard-shortcuts.ts

@@ -284,4 +284,7 @@ export const KEYBOARD_SHORTCUTS = {
   threads: [
     { key: "x", description: "shortcuts.threads.expand_collapse" },
   ],
+  composer: [
+    { key: "t", description: "shortcuts.composer.template_picker" },
+  ],
 } as const;