Productivity features, UI translations, local email translation

- Snooze (with a Snoozed folder), keyboard shortcuts with a Settings
  section, mail templates (Compose > More > Templates), a Subscriptions
  folder with bulk unsubscribe/delete, and tracker blocking with a
  Privacy toggle (on by default)
- Own dialogs instead of browser popups; emoji picker in compose; compose
  windows survive a visit to Settings
- Sentence completion learned from the user's sent mail (no AI)
- Interface in English and Polish (one file per language, English as the
  fallback), picked from the browser on first run, then kept
- Email translation on the server via LibreTranslate: Translate button
  and "Translate to...", messages in several languages translated part
  by part, names/addresses/emoji/file names/phone numbers kept, an
  animated line while translating
- Antispam "Mark as spam" loads with the message, not when the menu opens

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package.json

@@ -13,9 +13,11 @@
     "gen:api": "openapi-typescript http://127.0.0.1:8000/openapi.json -o src/api/schema.ts"
   },
   "dependencies": {
+    "@emoji-mart/data": "~1.2.1",
     "@tanstack/react-query": "~5.51.0",
     "@tanstack/react-virtual": "~3.8.0",
     "docx-preview": "^0.4.0",
+    "emoji-mart": "~5.6.0",
     "jszip": "^3.10.1",
     "pdfjs-dist": "^4.10.38",
     "qrcode.react": "~4.1.0",