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/routes.py

@@ -46,10 +46,6 @@ def setup_subscription_webapp_routes(app: web.Application) -> None:
         rf"{WEBAPP_FAVICON_PATH}/{{digest:[0-9a-f]{{16}}}}/{{filename:[A-Za-z0-9_.-]+}}",
         webapp_favicon_route,
     )
-    app.router.add_get(
-        r"/webapp-emoji/{codepoints:[0-9a-f_]+}/512.{ext:gif|webp}",
-        webapp_animated_emoji_route,
-    )
     app.router.add_get("/subscription_webapp.{asset_hash:[0-9a-f]{8}}.css", css_asset_route)
     app.router.add_get("/subscription_webapp.css", css_asset_route)
     app.router.add_get(