fix: back do android saia da sala em vez de fechar o viewer de imagem

O PopScope sozinho nao resolvia: GoRouterDelegate.popRoute() itera
_findCurrentNavigators(), que devolve [root, shell] e retorna
states.reversed, ou seja, o shell navigator e consultado primeiro. O
viewer e um showDialog no root navigator, entao o shell nunca ve o
PopEntry dele, popa a sala e retorna true antes de chegar no dialog.

BackButtonListener registra no BackButtonDispatcher raiz, que e
consultado antes do RouterDelegate.popRoute(), entao o viewer fecha
primeiro. Aplicado tambem no MxcImageViewer (avatar/sticker), que tinha
o mesmo defeito.

lib/pages/image_viewer/image_viewer.dart

@@ -199,15 +199,25 @@ class ImageViewerController extends State<ImageViewer> {
   }
 
   @override
-  Widget build(BuildContext context) => PopScope<Object?>(
+  Widget build(BuildContext context) => BackButtonListener(
-    // Claim the system back button (and predictive back gesture) for the
+    // A [PopScope] alone is not enough here: go_router's `popRoute()` asks
-    // viewer. Without this, the request can reach go_router instead, which
+    // `GoRouterDelegate._findCurrentNavigators()`, which returns the shell
-    // pops the chat room out of the shell and leaves the image on screen.
+    // navigator BEFORE the root one. The viewer is a dialog on the root
+    // navigator, so the shell navigator never sees its [PopEntry], pops the
+    // chat room instead and returns `true`, leaving the image on screen.
+    // A [BackButtonListener] is asked before the router gets the request at
+    // all, so the viewer gets to close itself first.
+    onBackButtonPressed: () async {
+      closeAction();
+      return true;
+    },
+    child: PopScope<Object?>(
       canPop: false,
       onPopInvokedWithResult: (didPop, _) {
         if (didPop) return;
         closeAction();
       },
       child: ImageViewerView(this),
+    ),
   );
 }

lib/widgets/mxc_image_viewer.dart

@@ -24,9 +24,16 @@ class MxcImageViewer extends StatelessWidget {
       backgroundColor: Colors.black.withAlpha(200),
       foregroundColor: Colors.white,
     );
-    return PopScope<Object?>(
+    return BackButtonListener(
-      // Claim the system back button for the viewer so that it is dismissed
+      // Same reason as the room image viewer: this dialog lives on the root
-      // instead of letting the router navigate away underneath it.
+      // navigator, but go_router asks the shell navigator first, so only a
+      // [BackButtonListener] (which runs before the router) can keep the back
+      // button from popping the page underneath.
+      onBackButtonPressed: () async {
+        _close(context);
+        return true;
+      },
+      child: PopScope<Object?>(
         canPop: false,
         onPopInvokedWithResult: (didPop, _) {
           if (didPop) return;
@@ -73,6 +80,7 @@ class MxcImageViewer extends StatelessWidget {
             ),
           ),
         ),
+      ),
     );
   }
 }