feat: image fullscreen viewer, public room browser

- Chat: tap any image to open fullscreen InteractiveViewer (pinch/zoom)
- Rooms: public room browser (Find rooms FAB + empty state button)
  searches Matrix room directory via queryPublicRooms, join from results
- Rooms: empty state now shows 'Find rooms' button to onboard new users

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

lib/features/chat/presentation/message_bubble.dart

@@ -1,4 +1,4 @@
-// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-10
+// Version: 1.3.0 | Created: 2026-04-01 | Updated: 2026-04-27
 // Message bubble widget. Handles text, images, files, redacted, replies.
 
 import 'package:cached_network_image/cached_network_image.dart';
@@ -172,11 +172,15 @@ class _ImageContent extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    if (message.mediaUrl != null) {
-      return ClipRRect(
+    final url = message.mediaUrl;
+    if (url == null) return const Icon(Icons.image_not_supported);
+
+    return GestureDetector(
+      onTap: () => _openFullscreen(context, url, message.body),
+      child: ClipRRect(
         borderRadius: BorderRadius.circular(8),
         child: CachedNetworkImage(
-          imageUrl: message.mediaUrl!,
+          imageUrl: url,
           width: 240,
           fit: BoxFit.cover,
           placeholder: (_, __) => const SizedBox(
@@ -188,9 +192,60 @@ class _ImageContent extends StatelessWidget {
             child: Center(child: Icon(Icons.broken_image)),
           ),
         ),
-      );
-    }
-    return const Icon(Icons.image_not_supported);
+      ),
+    );
+  }
+
+  void _openFullscreen(BuildContext context, String url, String? caption) {
+    Navigator.of(context).push(
+      MaterialPageRoute<void>(
+        fullscreenDialog: true,
+        builder: (_) => _ImageViewer(url: url, caption: caption),
+      ),
+    );
+  }
+}
+
+class _ImageViewer extends StatelessWidget {
+  const _ImageViewer({required this.url, this.caption});
+
+  final String url;
+  final String? caption;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      backgroundColor: Colors.black,
+      appBar: AppBar(
+        backgroundColor: Colors.black,
+        foregroundColor: Colors.white,
+        title: caption != null
+            ? Text(
+                caption!,
+                style: const TextStyle(fontSize: 14),
+                overflow: TextOverflow.ellipsis,
+              )
+            : null,
+      ),
+      body: Center(
+        child: InteractiveViewer(
+          minScale: 0.5,
+          maxScale: 4,
+          child: CachedNetworkImage(
+            imageUrl: url,
+            fit: BoxFit.contain,
+            placeholder: (_, __) => const CircularProgressIndicator(
+              color: Colors.white,
+            ),
+            errorWidget: (_, __, ___) => const Icon(
+              Icons.broken_image,
+              color: Colors.white,
+              size: 64,
+            ),
+          ),
+        ),
+      ),
+    );
   }
 }