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.

lib/screens/profile_screen.dart

@@ -1017,15 +1017,26 @@ class _ProfileScreenState extends State<ProfileScreen>
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
     const double headerHeight = 200.0;
-    const double cardTopOffset = 130.0; // card starts overlapping into header
+    const double cardTopOffset = 130.0;
+
+    Widget sectionTitle(String text) => Padding(
+          padding: const EdgeInsets.fromLTRB(18, 16, 18, 12),
+          child: Text(
+            text,
+            style: theme.textTheme.titleMedium
+                ?.copyWith(fontWeight: FontWeight.w600, fontSize: 18),
+          ),
+        );
 
     return Scaffold(
       backgroundColor: theme.scaffoldBackgroundColor,
-      body: SingleChildScrollView(
-        child: Column(
-          children: [
-            // Header + Profile Card overlap using Stack
-            Stack(
+      // CustomScrollView: only visible event cards are built — no full-tree Column renders
+      body: CustomScrollView(
+        physics: const BouncingScrollPhysics(),
+        slivers: [
+          // Header gradient + Profile card overlap (same visual as before)
+          SliverToBoxAdapter(
+            child: Stack(
               children: [
                 _buildGradientHeader(context, headerHeight),
                 Padding(
@@ -1034,13 +1045,74 @@ class _ProfileScreenState extends State<ProfileScreen>
                 ),
               ],
             ),
+          ),
 
-            // Event sections
-            _buildEventSections(context),
-
-            const SizedBox(height: 32),
+          // ── Ongoing Events ──
+          if (_ongoingEvents.isNotEmpty) ...[
+            SliverToBoxAdapter(child: sectionTitle('Ongoing Events')),
+            SliverPadding(
+              padding: const EdgeInsets.symmetric(horizontal: 18),
+              sliver: SliverList(
+                delegate: SliverChildBuilderDelegate(
+                  (ctx, i) => _eventListTileFromModel(_ongoingEvents[i]),
+                  childCount: _ongoingEvents.length,
+                ),
+              ),
+            ),
+            const SliverToBoxAdapter(child: SizedBox(height: 24)),
           ],
-        ),
+
+          // ── Upcoming Events ──
+          SliverToBoxAdapter(child: sectionTitle('Upcoming Events')),
+          if (_loadingEvents)
+            const SliverToBoxAdapter(child: SizedBox.shrink())
+          else if (_upcomingEvents.isEmpty)
+            SliverToBoxAdapter(
+              child: Padding(
+                padding: const EdgeInsets.symmetric(horizontal: 18, vertical: 12),
+                child: Text('No upcoming events',
+                    style: theme.textTheme.bodyMedium
+                        ?.copyWith(color: theme.hintColor)),
+              ),
+            )
+          else
+            SliverPadding(
+              padding: const EdgeInsets.symmetric(horizontal: 18),
+              sliver: SliverList(
+                delegate: SliverChildBuilderDelegate(
+                  (ctx, i) => _eventListTileFromModel(_upcomingEvents[i]),
+                  childCount: _upcomingEvents.length,
+                ),
+              ),
+            ),
+          const SliverToBoxAdapter(child: SizedBox(height: 24)),
+
+          // ── Past Events ──
+          SliverToBoxAdapter(child: sectionTitle('Past Events')),
+          if (_loadingEvents)
+            const SliverToBoxAdapter(child: SizedBox.shrink())
+          else if (_pastEvents.isEmpty)
+            SliverToBoxAdapter(
+              child: Padding(
+                padding: const EdgeInsets.symmetric(horizontal: 18, vertical: 12),
+                child: Text('No past events',
+                    style: theme.textTheme.bodyMedium
+                        ?.copyWith(color: theme.hintColor)),
+              ),
+            )
+          else
+            SliverPadding(
+              padding: const EdgeInsets.symmetric(horizontal: 18),
+              sliver: SliverList(
+                delegate: SliverChildBuilderDelegate(
+                  (ctx, i) => _eventListTileFromModel(_pastEvents[i], faded: true),
+                  childCount: _pastEvents.length,
+                ),
+              ),
+            ),
+
+          const SliverToBoxAdapter(child: SizedBox(height: 32)),
+        ],
       ),
     );
   }