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_completion_api.py

@@ -0,0 +1,41 @@
+"""Sentence completion over the API, from the user's sent mail."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from app.api import completion as api_completion
+from app.completion import build_index_job
+from tests.api.conftest import HDR, MailCtx, raw_mail
+
+
+async def _suggest(ctx: MailCtx, before: str) -> str | None:
+    resp = await ctx.client.post("/api/completion", json={"before": before}, headers=HDR)
+    assert resp.status_code == 200, resp.text
+    return resp.json()["suggestion"]
+
+
+async def test_suggests_from_sent_mail(mail_ctx: MailCtx, monkeypatch: pytest.MonkeyPatch) -> None:
+    queued: list[tuple[Any, ...]] = []
+
+    async def fake_enqueue(name: str, *args: Any, **kwargs: Any) -> None:
+        queued.append((name, *args))
+
+    monkeypatch.setattr(api_completion, "enqueue_job", fake_enqueue)
+    body = "Hi Anna,\nThe renders will be ready by Thursday evening.\n\nOn Mon Anna wrote:\n> ok"
+    for uid in (1, 2):
+        await mail_ctx.deliver(
+            raw_mail("Renders", sender="me@example.com").replace(b"Renders body", body.encode()),
+            uid,
+            folder="sent",
+        )
+
+    # No index yet: a build is asked for, and the starter phrases still answer.
+    assert await _suggest(mail_ctx, "I look forward ") == "to hearing from you."
+    assert queued and queued[0][0] == "build_index_job"
+
+    assert await build_index_job({}, str(mail_ctx.user_id)) > 0
+    assert await _suggest(mail_ctx, "The renders will be re") == "ady by Thursday evening."
+    assert await _suggest(mail_ctx, "> ok") is None  # quoted text was not learned