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/calendar_screen.dart

@@ -1,6 +1,7 @@
 // lib/screens/calendar_screen.dart
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
+import 'package:cached_network_image/cached_network_image.dart';
 import '../features/events/services/events_service.dart';
 import '../features/events/models/event_models.dart';
 import 'learn_more_screen.dart';
@@ -511,7 +512,16 @@ class _CalendarScreenState extends State<CalendarScreen> {
           children: [
             ClipRRect(
               borderRadius: const BorderRadius.vertical(top: Radius.circular(14)),
-              child: imgUrl != null ? Image.network(imgUrl, height: 150, width: double.infinity, fit: BoxFit.cover, errorBuilder: (_, __, ___) => Container(height: 150, color: theme.dividerColor)) : Container(height: 150, color: theme.dividerColor, child: Icon(Icons.event, size: 44, color: theme.hintColor)),
+              child: imgUrl != null
+                  ? CachedNetworkImage(
+                      imageUrl: imgUrl,
+                      height: 150,
+                      width: double.infinity,
+                      fit: BoxFit.cover,
+                      placeholder: (_, __) => Container(height: 150, color: theme.dividerColor),
+                      errorWidget: (_, __, ___) => Container(height: 150, color: theme.dividerColor),
+                    )
+                  : Container(height: 150, color: theme.dividerColor, child: Icon(Icons.event, size: 44, color: theme.hintColor)),
             ),
             Padding(
               padding: const EdgeInsets.fromLTRB(12, 12, 12, 14),