perf: fix remaining 11 performance issues across 5 screens

Critical — Image.network → CachedNetworkImage:
- home_screen.dart: hero/carousel banner image now cached with placeholder
- profile_screen.dart: avatar and event list tile images now cached
- calendar_screen.dart: event card images now cached with placeholder

High:
- profile_screen.dart: TextEditingControllers in dialogs now properly
  disposed via .then() and after await to prevent memory leaks

Medium:
- search_screen.dart: shrinkWrap:true → ConstrainedBox(maxHeight:320) +
  ClampingScrollPhysics for smooth search result scrolling
- learn_more_screen.dart: MediaQuery.of(context) cached once per method
  instead of being called multiple times on every frame

lib/screens/home_screen.dart

@@ -1216,10 +1216,12 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                 child: SizedBox(
                   width: double.infinity,
                   child: img != null && img.isNotEmpty
-                      ? Image.network(
-                          img,
+                      ? CachedNetworkImage(
+                          imageUrl: img,
                           fit: BoxFit.cover,
-                          errorBuilder: (context, error, stackTrace) =>
+                          placeholder: (_, __) => Container(
+                              decoration: const BoxDecoration(color: Color(0xFF1A2A4A))),
+                          errorWidget: (_, __, ___) =>
                               Container(decoration: AppDecoration.blueGradientRounded(radius)),
                         )
                       : Container(