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The web build hung on the native splash because the asset manifest could not
be published: "assets/icons/splash_orb.png" is 27 characters, so its length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/lib/Grounded/designs/scene/ScenePainter.dart

@@ -11,7 +11,10 @@ class ScenePainter extends CustomPainter {
 
   final int seed;
 
-  ScenePainter(this.palette, this.seed);
+  /// 1 → the sun sits in its place; 0 → it is down behind the ridges.
+  final double sunLift;
+
+  ScenePainter(this.palette, this.seed, {this.sunLift = 1});
 
   @override
   void paint(Canvas canvas, Size size) {
@@ -36,7 +39,7 @@ class ScenePainter extends CustomPainter {
     final double radius = math.min(w, h) * (0.13 + random.nextDouble() * 0.05);
     final Offset sun = Offset(
       w * (0.18 + random.nextDouble() * 0.64),
-      h * (0.26 + random.nextDouble() * 0.12),
+      h * (0.26 + random.nextDouble() * 0.12) + h * 0.5 * (1 - sunLift),
     );
     canvas.drawCircle(
       sun,
@@ -109,6 +112,8 @@ class ScenePainter extends CustomPainter {
 
   @override
   bool shouldRepaint(covariant ScenePainter oldDelegate) {
-    return oldDelegate.seed != seed || oldDelegate.palette != palette;
+    return oldDelegate.seed != seed ||
+        oldDelegate.palette != palette ||
+        oldDelegate.sunLift != sunLift;
   }
 }