Animate the splash and fix the web preview hanging on the logo

The splash now raises the sun in the mark and over a painted valley, floats
four feature chips around the mark, shimmers the wordmark, shows a three-beat
"how it works" card, drifts light motes upward and walks a status line
through the session check. A tap skips it.

The web build hung on the native splash because the asset manifest could not
be published: "assets/icons/splash_orb.png" is 27 characters, so its length
prefix in AssetManifest.bin is an ESC byte. Renamed it to splash_mark.png and
removed the unused anchor assets.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/web/index.html

@@ -34,6 +34,8 @@
   <meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no" name="viewport">
   
   
+  
+  
   <style id="splash-screen-style">
     html {
       height: 100%
@@ -108,6 +110,7 @@
   </picture>
   
   
+  
   <!--
     You can customize the "flutter_bootstrap.js" script.
     This is useful to provide a custom configuration to the Flutter loader