Fix RmlUi CSS compatibility issues in store, settings, and music apps

- Replace linear-gradient with solid colors (not supported in RmlUi)
- Replace border shorthand with border-width/border-color properties
- Replace display:grid with flexbox in music quick-access cards
- Remove unsupported transition property
- Fix border-radius: 50% to use 9999px in theme.rcss
- Fix third-party app icon/label sizes to match system apps (72x72px, 16px)
- Replace data-model bindings with static values in settings

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main/assets/apps/store/store.rml

@@ -67,7 +67,7 @@
         .featured-banner {
             margin: 0 16px 16px 16px;
             height: 160px;
-            background: linear-gradient(135deg, #BB86FC 0%, #6200EE 100%);
+            background-color: #7C3AED;
             border-radius: 16px;
             padding: 20px;
             display: flex;
@@ -226,7 +226,8 @@
         .install-btn.installed {
             background-color: transparent;
             color: #BB86FC;
-            border: 1px solid #BB86FC;
+            border-width: 1px;
+            border-color: #BB86FC;
         }
 
         /* Category Chips */
@@ -374,7 +375,6 @@
             height: 100%;
             background-color: #BB86FC;
             border-radius: 2px;
-            transition: width 0.3s ease;
         }
 
         /* Toast */