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/widgets/mxc_image_viewer.dart

@@ -24,49 +24,57 @@ class MxcImageViewer extends StatelessWidget {
       backgroundColor: Colors.black.withAlpha(200),
       foregroundColor: Colors.white,
     );
-    return PopScope<Object?>(
-      // Claim the system back button for the viewer so that it is dismissed
-      // instead of letting the router navigate away underneath it.
-      canPop: false,
-      onPopInvokedWithResult: (didPop, _) {
-        if (didPop) return;
+    return BackButtonListener(
+      // Same reason as the room image viewer: this dialog lives on the root
+      // 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: GestureDetector(
-        onTap: () => _close(context),
-        child: Scaffold(
-          backgroundColor: Colors.black.withAlpha(128),
-          extendBodyBehindAppBar: true,
-          appBar: AppBar(
-            elevation: 0,
-            leading: IconButton(
-              style: iconButtonStyle,
-              icon: const Icon(Icons.close),
-              onPressed: () => _close(context),
-              color: Colors.white,
-              tooltip: L10n.of(context).close,
+      child: PopScope<Object?>(
+        canPop: false,
+        onPopInvokedWithResult: (didPop, _) {
+          if (didPop) return;
+          _close(context);
+        },
+        child: GestureDetector(
+          onTap: () => _close(context),
+          child: Scaffold(
+            backgroundColor: Colors.black.withAlpha(128),
+            extendBodyBehindAppBar: true,
+            appBar: AppBar(
+              elevation: 0,
+              leading: IconButton(
+                style: iconButtonStyle,
+                icon: const Icon(Icons.close),
+                onPressed: () => _close(context),
+                color: Colors.white,
+                tooltip: L10n.of(context).close,
+              ),
+              backgroundColor: Colors.transparent,
             ),
-            backgroundColor: Colors.transparent,
-          ),
-          body: InteractiveViewer(
-            minScale: 1.0,
-            maxScale: 10.0,
-            onInteractionEnd: (endDetails) {
-              if (endDetails.velocity.pixelsPerSecond.dy >
-                  MediaQuery.sizeOf(context).height * 1.5) {
-                _close(context);
-              }
-            },
-            child: Center(
-              child: GestureDetector(
-                // Ignore taps to not go back here:
-                onTap: () {},
-                child: MxcImage(
-                  key: ValueKey(mxContent.toString()),
-                  uri: mxContent,
-                  fit: BoxFit.contain,
-                  isThumbnail: false,
-                  animated: true,
+            body: InteractiveViewer(
+              minScale: 1.0,
+              maxScale: 10.0,
+              onInteractionEnd: (endDetails) {
+                if (endDetails.velocity.pixelsPerSecond.dy >
+                    MediaQuery.sizeOf(context).height * 1.5) {
+                  _close(context);
+                }
+              },
+              child: Center(
+                child: GestureDetector(
+                  // Ignore taps to not go back here:
+                  onTap: () {},
+                  child: MxcImage(
+                    key: ValueKey(mxContent.toString()),
+                    uri: mxContent,
+                    fit: BoxFit.contain,
+                    isThumbnail: false,
+                    animated: true,
+                  ),
                 ),
               ),
             ),