refactor(splash): convert Skip to explicit guest login flow

- Splash Skip button now dispatches `AuthGuestLoginSubmitted` instead of
  direct routing, keeping guest auth under the state machine
- Update PWA install dialog copy ("Add to Home Screen") and drop favicon
  entry from manifest icons
- Add debug logging and routing sequence docs to splash-button

public/manifest.json

@@ -10,12 +10,6 @@
   "theme_color": "#f84d96",
   "lang": "en",
   "icons": [
-    {
-      "src": "/favicon.ico",
-      "sizes": "any",
-      "type": "image/x-icon",
-      "purpose": "any"
-    },
     {
       "src": "/images/splash/Icon-192.png",
       "sizes": "192x192",