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/config.py

@@ -71,6 +71,9 @@ class Settings(BaseSettings):
     # Spelling and grammar checking in the compose window (LanguageTool server,
     # e.g. http://languagetool:8010). Empty: the check is off for everyone.
     languagetool_url: str = Field(default="", alias="LANGUAGETOOL_URL")
+    # Email translation: a LibreTranslate server (app/translation.py), e.g.
+    # http://libretranslate:5000. Empty: translation is off.
+    libretranslate_url: str = Field(default="", alias="LIBRETRANSLATE_URL")
     # New passwords are checked against known data breaches (Have I Been Pwned's
     # k-anonymity range API: only 5 hex digits of a SHA-1 leave the server).
     password_breach_check: bool = Field(default=True, alias="PASSWORD_BREACH_CHECK")