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/migrations/versions/0029_completion.py

@@ -0,0 +1,49 @@
+"""sentence completion: a per-user index of phrases from their sent mail
+
+``completion_phrases`` maps the words before the cursor (normalized) to what
+the user usually writes next, with how often; rebuilt from the sent mail at
+most daily (app/completion.py). The primary key leads with (user_id, context),
+which is the lookup.
+
+Revision ID: 0029
+Revises: 0028
+Create Date: 2026-10-18
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0029"
+down_revision: str | None = "0028"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute(
+        """
+        CREATE TABLE completion_phrases (
+          user_id uuid NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+          context text NOT NULL,
+          continuation text NOT NULL,
+          count integer NOT NULL,
+          PRIMARY KEY (user_id, context, continuation)
+        )
+        """
+    )
+    op.execute(
+        """
+        CREATE TABLE completion_index_state (
+          user_id uuid PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+          built_at timestamptz NOT NULL
+        )
+        """
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP TABLE IF EXISTS completion_index_state")
+    op.execute("DROP TABLE IF EXISTS completion_phrases")