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/completion.py

@@ -0,0 +1,44 @@
+"""Sentence completion for the compose window (app/completion.py)."""
+
+from __future__ import annotations
+
+import datetime as dt
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel, Field
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app import completion
+from app.db import get_session
+from app.identity.deps import CurrentUser, require_csrf
+from app.models import CompletionIndexState
+from app.queue import enqueue_job
+
+router = APIRouter(prefix="/completion", tags=["compose"], dependencies=[Depends(require_csrf)])
+
+DbSession = Annotated[AsyncSession, Depends(get_session)]
+
+
+class CompletionIn(BaseModel):
+    # The text before the cursor (its last line is all that matters). A POST,
+    # not a query string: what the user is typing stays out of access logs.
+    before: str = Field(max_length=2000)
+
+
+class CompletionOut(BaseModel):
+    suggestion: str | None
+
+
+@router.post("", response_model=CompletionOut)
+async def suggest(body: CompletionIn, user: CurrentUser, db: DbSession) -> CompletionOut:
+    before = body.before[-300:]
+    built = await db.scalar(
+        select(CompletionIndexState.built_at).where(CompletionIndexState.user_id == user.id)
+    )
+    if built is None or built < dt.datetime.now(dt.UTC) - completion.REBUILD_AFTER:
+        # In the background; until it is built, the starter phrases still answer.
+        await enqueue_job("build_index_job", str(user.id), _job_id=f"complete:{user.id}")
+    personal = await completion.personal_phrases(db, user.id, completion.keys_for(before))
+    return CompletionOut(suggestion=completion.complete(personal, before))