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/app/web/webapp/application.py

@@ -33,7 +33,6 @@ def create_subscription_webapp_application(
     async def _startup(app_obj: web.Application) -> None:
         await _ensure_shared_http_session()
         await _warm_webapp_logo_cache(app_obj)
-        await _warm_webapp_animated_emoji_cache(app_obj)
         await warm_subscription_guides_config(app_obj)
 
     async def _shutdown(app_obj: web.Application) -> None: