feat: add shimmer loading skeletons for various pages and components

- Implemented UiShimmer as a core shimmer wrapper for animated gradient effects.
- Created shimmer presets for list items, stats cards, section headers, and more.
- Developed specific skeletons for billing, invoices, coverage, hubs, reports, payments, shifts, and home pages.
- Enhanced user experience by providing visual placeholders during data loading.

apps/mobile/pubspec.lock

@@ -1397,6 +1397,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.0"
+  shimmer:
+    dependency: transitive
+    description:
+      name: shimmer
+      sha256: "5f88c883a22e9f9f299e5ba0e4f7e6054857224976a5d9f839d4ebdc94a14ac9"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.0"
   sky_engine:
     dependency: transitive
     description: flutter