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/learn_more_screen.dart

@@ -223,9 +223,10 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> {
       );
     }
 
-    final screenHeight = MediaQuery.of(context).size.height;
+    final mediaQuery = MediaQuery.of(context);
+    final screenHeight = mediaQuery.size.height;
     final imageHeight = screenHeight * 0.45;
-    final topPadding = MediaQuery.of(context).padding.top;
+    final topPadding = mediaQuery.padding.top;
 
     return Scaffold(
       backgroundColor: theme.scaffoldBackgroundColor,
@@ -355,6 +356,7 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> {
   // ---------------------------------------------------------------------------
 
   Widget _buildLoadingShimmer(ThemeData theme) {
+    final shimmerHeight = MediaQuery.of(context).size.height;
     return SafeArea(
       child: Padding(
         padding: const EdgeInsets.all(20),
@@ -363,7 +365,7 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> {
           children: [
             // Placeholder image
             Container(
-              height: MediaQuery.of(context).size.height * 0.42,
+              height: shimmerHeight * 0.42,
               decoration: BoxDecoration(
                 color: theme.dividerColor.withOpacity(0.3),
                 borderRadius: BorderRadius.circular(28),