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/0030_translation.py

@@ -0,0 +1,45 @@
+"""email translation: a message's detected language, and translations kept
+
+``messages.content_language``: NULL until the message is first shown, '' when
+the language could not be told. ``message_translations``: one translated HTML
+per message, target language and remote-images choice (app/translation.py);
+dropped with the message.
+
+Revision ID: 0030
+Revises: 0029
+Create Date: 2026-10-18
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0030"
+down_revision: str | None = "0029"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE messages ADD COLUMN content_language text")
+    op.execute(
+        """
+        CREATE TABLE message_translations (
+          message_id uuid NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
+          target text NOT NULL,
+          images boolean NOT NULL,
+          source text NOT NULL,
+          html text NOT NULL,
+          sanitizer_v integer NOT NULL,
+          created_at timestamptz NOT NULL DEFAULT now(),
+          PRIMARY KEY (message_id, target, images)
+        )
+        """
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP TABLE IF EXISTS message_translations")
+    op.execute("ALTER TABLE messages DROP COLUMN IF EXISTS content_language")