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>

backend/tests/api/test_api.py

@@ -391,7 +391,7 @@ async def test_folder_counts(api: ApiCtx) -> None:
     # All three seeded inbox messages are unread; no drafts yet.
     resp = await api.client.get("/api/mailboxes/counts")
     assert resp.status_code == 200
-    assert resp.json() == {"inbox_unread": 3, "drafts": 0}
+    assert resp.json() == {"inbox_unread": 3, "drafts": 0, "snoozed": 0}
 
     # Marking one message \Seen lowers the unread count.
     msg = await api.session.get(Message, api.message_ids[0])