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/profile_screen.dart

@@ -1,5 +1,6 @@
 import 'dart:io';
 
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:image_picker/image_picker.dart';
@@ -273,6 +274,7 @@ class _ProfileScreenState extends State<ProfileScreen>
     final result = await showDialog<String?>(
       context: context,
       builder: (ctx) {
+        // Note: ctl is disposed after dialog closes below
         final theme = Theme.of(ctx);
         return AlertDialog(
           title: const Text('Enter image path or URL'),
@@ -305,6 +307,7 @@ class _ProfileScreenState extends State<ProfileScreen>
       },
     );
 
+    ctl.dispose();
     if (result == null || result.isEmpty) return;
     await _saveProfile(_username, _email, result);
   }
@@ -318,6 +321,7 @@ class _ProfileScreenState extends State<ProfileScreen>
       isScrollControlled: true,
       backgroundColor: Colors.transparent,
       builder: (ctx) {
+        // nameCtl and emailCtl are disposed via .then() below
         final theme = Theme.of(ctx);
         return DraggableScrollableSheet(
           expand: false,
@@ -419,7 +423,10 @@ class _ProfileScreenState extends State<ProfileScreen>
           },
         );
       },
-    );
+    ).then((_) {
+      nameCtl.dispose();
+      emailCtl.dispose();
+    });
   }
 
   // ───────── Avatar builder (reused, with size param) ─────────
@@ -428,11 +435,14 @@ class _ProfileScreenState extends State<ProfileScreen>
     final path = _profileImage.trim();
     if (path.startsWith('http')) {
       return ClipOval(
-          child: Image.network(path,
+          child: CachedNetworkImage(
+              imageUrl: path,
               width: size,
               height: size,
               fit: BoxFit.cover,
-              errorBuilder: (_, __, ___) =>
+              placeholder: (_, __) =>
+                  Container(width: size, height: size, color: const Color(0xFFE5E7EB)),
+              errorWidget: (_, __, ___) =>
                   Icon(Icons.person, size: size / 2, color: Colors.grey)));
     }
     if (kIsWeb) {
@@ -497,11 +507,16 @@ class _ProfileScreenState extends State<ProfileScreen>
         if (imageUrl.startsWith('http')) {
           return ClipRRect(
               borderRadius: BorderRadius.circular(12),
-              child: Image.network(imageUrl,
+              child: CachedNetworkImage(
+                  imageUrl: imageUrl,
                   width: 60,
                   height: 60,
                   fit: BoxFit.cover,
-                  errorBuilder: (_, __, ___) => Container(
+                  placeholder: (_, __) => Container(
+                      width: 60,
+                      height: 60,
+                      color: const Color(0xFFE5E7EB)),
+                  errorWidget: (_, __, ___) => Container(
                       width: 60,
                       height: 60,
                       color: theme.dividerColor,