refactor(webapp): remove emoji web app logo option

Drop the emoji-logo feature (and its font picker) from the Web App. Only
an uploaded/linked image logo and favicon remain; when no logo is set,
the default project logo is shown. Existing emoji-logo overrides are
ignored — the keys are gone from the manifest, so the override service
skips them and the app falls back to the default logo.

- Remove WEBAPP_LOGO_USE_EMOJI / WEBAPP_LOGO_EMOJI / WEBAPP_LOGO_EMOJI_FONT
  settings, validators, manifest entries and override/runtime plumbing.
- Strip the animated-emoji fetch/cache subsystem, the /webapp-emoji route
  and emoji branches from logo/favicon resolution; leftover emoji cache
  files are now purged on appearance save.
- Simplify BrandMark to an image-only component and drop the emoji UI
  from the admin Appearance section.
- Regenerate the demo settings manifest and clean docs, locales, nginx
  and demo data of emoji-logo references.

backend/bot/services/email_templates.py

@@ -48,8 +48,6 @@ def _safe_color(value: Optional[str]) -> str:
 def _public_logo_url(settings: Settings) -> Optional[str]:
     """Email recipients can't reach the in-app /webapp-logo proxy, so only a
     stored public https URL can be used directly. Anything else is dropped."""
-    if getattr(settings, "WEBAPP_LOGO_USE_EMOJI", False):
-        return None
     raw = (settings.WEBAPP_LOGO_URL or "").strip()
     if not raw:
         return None