perf: fix Android lag, snapping animations & slow image loading

Fix 1: Replace overshooting Cubic(1.95) tab glider curve with
  Curves.easeInOutCubic; reduce duration 450ms → 280ms
Fix 2: Replace marquee jumpTo() with animateTo(linear) for fluid scroll
Fix 3: Replace Image.network with CachedNetworkImage in search results
Fix 4: Replace Image.network with CachedNetworkImage in desktop cards
Fix 5: Wrap IndexedStack children in RepaintBoundary to isolate
  repaints across tabs (Home/Calendar/Contribute/Profile)
Fix 6: Replace setState on PageView.onPageChanged with ValueNotifier
  so only the carousel dots widget rebuilds on swipe
Fix 7: Wrap animated tab glider in RepaintBoundary
Fix 8: Replace shrinkWrap:true ListView with ConstrainedBox(maxHeight)
  to eliminate O(n) layout pass in search results
Fix 9: Increase image cache to 200MB / 500 images in main()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/contribute_screen.dart

@@ -1357,7 +1357,7 @@ class _ContributeScreenState extends State<ContributeScreen>
   // ─────────────────────────────────────────────────────────────────────────
   // Segmented Tabs (4 tabs)
   // ─────────────────────────────────────────────────────────────────────────
-  static const Curve _bouncyCurve = Cubic(0.37, 1.95, 0.66, 0.56);
+  static const Curve _bouncyCurve = Curves.easeInOutCubic;
 
   static const List<IconData> _tabIcons = [
     Icons.edit_outlined,
@@ -1376,7 +1376,8 @@ class _ContributeScreenState extends State<ContributeScreen>
           const double padding = 5.0;
           final tabWidth = (constraints.maxWidth - padding * 2) / tabs.length;
 
-          return ClipRRect(
+          return RepaintBoundary(
+            child: ClipRRect(
             borderRadius: BorderRadius.circular(16),
             child: Container(
               height: 52,
@@ -1389,7 +1390,7 @@ class _ContributeScreenState extends State<ContributeScreen>
                 children: [
                   // Sliding glider
                   AnimatedPositioned(
-                    duration: const Duration(milliseconds: 450),
+                    duration: const Duration(milliseconds: 280),
                     curve: _bouncyCurve,
                     left: padding + _activeTab * tabWidth,
                     top: padding,
@@ -1444,7 +1445,7 @@ class _ContributeScreenState extends State<ContributeScreen>
                 ],
               ),
             ),
-          );
+          )); // RepaintBoundary
         },
       ),
     );