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The web build hung on the native splash because the asset manifest could not
be published: "assets/icons/splash_orb.png" is 27 characters, so its length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

@@ -26,6 +26,9 @@ class SceneArt extends StatelessWidget {
   /// Darkens the lower half so white type laid over it stays legible.
   final bool shade;
 
+  /// Lowers the sun behind the ridges; animate it for a sunrise.
+  final double sunLift;
+
   const SceneArt({
     super.key,
     required this.palette,
@@ -34,6 +37,7 @@ class SceneArt extends StatelessWidget {
     this.figureScale = 0.5,
     this.figureAlignment = const Alignment(0.35, 0.15),
     this.shade = false,
+    this.sunLift = 1,
   });
 
   @override
@@ -50,7 +54,8 @@ class SceneArt extends StatelessWidget {
           child: Stack(
             fit: StackFit.expand,
             children: [
-              CustomPaint(painter: ScenePainter(palette, seed)),
+              CustomPaint(
+                  painter: ScenePainter(palette, seed, sunLift: sunLift)),
               if (figure != null)
                 Align(
                   alignment: figureAlignment,