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.

tests/test_webapp_route_contract.py

@@ -90,7 +90,6 @@ class WebAppRouteContractTests(unittest.TestCase):
             ("GET", "/robots.txt"): "robots_txt_route",
             ("GET", "/webapp-logo"): "webapp_logo_route",
             ("GET", "/webapp-uploaded-logo/{filename}"): "webapp_uploaded_logo_route",
-            ("GET", "/webapp-emoji/{codepoints}/512.{ext}"): "webapp_animated_emoji_route",
             ("GET", "/subscription_webapp.css"): "css_asset_route",
             ("GET", "/subscription_webapp_admin.css"): "admin_css_asset_route",
             ("GET", "/webapp-theme-css/{path}"): "theme_css_asset_route",