Translation on Firefox Translations models; new logo

- Email translation moves from LibreTranslate to our own translator
  service (translator/): Mozilla's Firefox Translations models on the
  Bergamot engine, fastText language detection (176 languages). Polish,
  English, German, French, Spanish and Italian on first start (~350 MB
  instead of ~10 GB); Admin panel > Email translation installs and
  removes languages. Built and pushed as the `translator` image.
- Every message is translated paragraph by paragraph as plain text;
  names travel with their sentence and are kept only where the model
  changed them; a sentence start the model copied ("Enclosed") is tried
  again in lower case; list markers are not sent
- The Translate button shows only for installed languages
- New logo: favicon, app/maskable/iPhone icons, notification badge,
  email logo and calendar copy regenerated; BIMI logo rebuilt in SVG
  Tiny PS with its gradients; masks render in browsers
- Launch screen background is the app's dark grey

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/translation.py

@@ -25,6 +25,6 @@ class TranslationInfoOut(BaseModel):
 async def translation_languages(user: CurrentUser) -> TranslationInfoOut:
     found = await translation.languages()
     return TranslationInfoOut(
-        available=bool(found),
+        available=len(found) > 1,  # English alone translates nothing
         languages=[LanguageOut(code=x.code, name=x.name) for x in found],
     )