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

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

lib/screens/search_screen.dart

@@ -305,9 +305,11 @@ class _SearchScreenState extends State<SearchScreen> {
                     child: Center(child: Text('No results found', style: TextStyle(color: Colors.grey[500]))),
                   )
                 else
-                  ListView.separated(
-                    shrinkWrap: true,
-                    physics: const NeverScrollableScrollPhysics(),
+                  ConstrainedBox(
+                    constraints: const BoxConstraints(maxHeight: 320),
+                    child: ListView.separated(
+                    shrinkWrap: false,
+                    physics: const ClampingScrollPhysics(),
                     itemCount: _searchResults.length,
                     separatorBuilder: (_, __) => Divider(color: Colors.grey[200], height: 1),
                     itemBuilder: (ctx, idx) {
@@ -326,6 +328,7 @@ class _SearchScreenState extends State<SearchScreen> {
                       );
                     },
                   ),
+                  ),
               ] else ...[
                 const Text('Popular Cities', style: TextStyle(fontWeight: FontWeight.bold, fontSize: 16, color: Color(0xFF1A1A2E))),
                 const SizedBox(height: 12),