feat: responsive layout, date filtering, location search, calendar fix

- Make web responsive layout use width-based mobile detection (820px)
- Add date filter chips that actually filter events by date ranges
- Custom calendar dialog with event dots on Date chip tap
- Update location search with Kerala cities and pincode display
- Fix calendar screen overflow errors and broken event indicators
- Replace thumbnail indicators with clean colored dots

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

lib/screens/responsive_layout.dart

@@ -21,7 +21,7 @@ class ResponsiveLayout extends StatelessWidget {
     Key? key,
     required this.mobile,
     required this.desktop,
-    this.mobileBreakpoint = 700, // tune this value if you prefer different breakpoint
+    this.mobileBreakpoint = 820, // consistent with MyApp.desktopBreakpoint
   })  : assert(mobileBreakpoint > 0),
         super(key: key);
 
@@ -35,12 +35,18 @@ class ResponsiveLayout extends StatelessWidget {
   bool _chooseMobile(BuildContext context) {
     final width = MediaQuery.of(context).size.width;
 
+    // On web, use width to determine mobile vs desktop so narrow browser
+    // windows (or mobile-sized preview) get the mobile UI.
+    if (kIsWeb) {
+      return width < mobileBreakpoint;
+    }
+
     // If running on Android/iOS, allow width to determine mobile vs desktop.
     if (_isMobilePlatform()) {
       return width < mobileBreakpoint;
     }
 
-    // On desktop platforms (Windows/macOS/Linux) and on web, always use desktop UI.
+    // On native desktop platforms (Windows/macOS/Linux) always use desktop UI.
     return false;
   }