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/_runtime.py

@@ -82,7 +82,6 @@ WEBAPP_DEFAULT_LOGO_PATH = "/webapp-default-logo.webp"
 WEBAPP_DEFAULT_FAVICON_DIGEST = "19b2a242e5b7bc2d"
 WEBAPP_DEFAULT_FAVICON_DIR = WEBAPP_DEFAULT_BRAND_DIR / "favicons" / WEBAPP_DEFAULT_FAVICON_DIGEST
 WEBAPP_DEFAULT_FAVICON_URL = f"{WEBAPP_FAVICON_PATH}/{WEBAPP_DEFAULT_FAVICON_DIGEST}/icon-180.png"
-WEBAPP_EMOJI_CACHE_DIR = APP_ROOT / "data" / "webapp-emoji"
 WEBAPP_CONFIG_PLACEHOLDER = "<!-- WEBAPP_CONFIG_SCRIPT -->"
 WEBAPP_I18N_PLACEHOLDER = "<!-- WEBAPP_I18N_SCRIPT -->"
 WEBAPP_JS_PLACEHOLDER = "<!-- WEBAPP_JS_SCRIPT -->"
@@ -92,7 +91,6 @@ DEV_MOCK_END_MARKER = "<!-- WEBAPP_DEV_MOCK_END -->"
 WEBAPP_RATE_LIMIT_WINDOW_SECONDS = 60
 WEBAPP_RATE_LIMIT_MAX_REQUESTS = 30
 WEBAPP_LOGO_MAX_BYTES = 2 * 1024 * 1024
-WEBAPP_EMOJI_MAX_BYTES = 4 * 1024 * 1024
 WEBAPP_THEME_CSS_MAX_BYTES = 512 * 1024
 WEBAPP_THEME_ASSET_MAX_BYTES = 1024 * 1024
 WEBAPP_THEME_ASSET_CONTENT_TYPES = {