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?>(
-    // Claim the system back button (and predictive back gesture) for the
-    // viewer. Without this, the request can reach go_router instead, which
-    // pops the chat room out of the shell and leaves the image on screen.
-    canPop: false,
-    onPopInvokedWithResult: (didPop, _) {
-      if (didPop) return;
+  Widget build(BuildContext context) => BackButtonListener(
+    // A [PopScope] alone is not enough here: go_router's `popRoute()` asks
+    // `GoRouterDelegate._findCurrentNavigators()`, which returns the shell
+    // 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: ImageViewerView(this),
+    child: PopScope<Object?>(
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) {
+        if (didPop) return;
+        closeAction();
+      },
+      child: ImageViewerView(this),
+    ),
   );
 }