perf: fix scroll lag on profile/contribute, unpin calendar gradient

profile_screen: SingleChildScrollView + Column eagerly built every event
card (all images, shadows, tiles) at once even when off-screen. Replaced
with CustomScrollView + SliverList so only visible tiles are built per
frame. Also switches to BouncingScrollPhysics for natural momentum.

contribute_screen: Each _formCard wrapped in RepaintBoundary so form
cards are isolated render layers — one card's repaint doesn't invalidate
its siblings. Added BouncingScrollPhysics to the form SingleChildScrollView.

calendar_screen: Blue gradient banner was Positioned(top:0) making it
sticky even as the user scrolled. Removed the fixed Positioned layer and
moved the gradient inside the CustomScrollView as the first sliver in a
Stack alongside the calendar card (which keeps its y=110 visual overlap).
Now the entire page — gradient, calendar, events — scrolls as one unit.

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

lib/screens/contribute_screen.dart

@@ -1470,6 +1470,7 @@ class _ContributeScreenState extends State<ContributeScreen>
   Widget _buildContributeTab(BuildContext context, GamificationProvider provider) {
     final theme = Theme.of(context);
     return SingleChildScrollView(
+      physics: const BouncingScrollPhysics(),
       padding: const EdgeInsets.fromLTRB(16, 20, 16, 32),
       child: Form(
         key: _formKey,
@@ -1563,13 +1564,15 @@ class _ContributeScreenState extends State<ContributeScreen>
   }
 
   Widget _formCard(List<Widget> children) {
-    return Container(
-      decoration: BoxDecoration(
-        color: Colors.white,
-        borderRadius: BorderRadius.circular(14),
-        boxShadow: [BoxShadow(color: Colors.black.withOpacity(0.04), blurRadius: 8, offset: const Offset(0, 2))],
+    return RepaintBoundary(
+      child: Container(
+        decoration: BoxDecoration(
+          color: Colors.white,
+          borderRadius: BorderRadius.circular(14),
+          boxShadow: [BoxShadow(color: Colors.black.withOpacity(0.04), blurRadius: 8, offset: const Offset(0, 2))],
+        ),
+        child: Column(children: children),
       ),
-      child: Column(children: children),
     );
   }