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/app/api/translation.py

@@ -0,0 +1,30 @@
+"""What email translation offers (app/translation.py), for the reading pane."""
+
+from __future__ import annotations
+
+from fastapi import APIRouter
+from pydantic import BaseModel
+
+from app import translation
+from app.identity.deps import CurrentUser
+
+router = APIRouter(prefix="/translation", tags=["mail"])
+
+
+class LanguageOut(BaseModel):
+    code: str
+    name: str
+
+
+class TranslationInfoOut(BaseModel):
+    available: bool
+    languages: list[LanguageOut]
+
+
+@router.get("/languages", response_model=TranslationInfoOut)
+async def translation_languages(user: CurrentUser) -> TranslationInfoOut:
+    found = await translation.languages()
+    return TranslationInfoOut(
+        available=bool(found),
+        languages=[LanguageOut(code=x.code, name=x.name) for x in found],
+    )