feat(app): crop cover & banner before upload (crop_your_image, 16:9 / 3:1)

review fixes: drop GIF from pre-sniff (package maps gif→png decode → never-ready hang); PopScope blocks back during processing

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

apps/mobile/pubspec.lock

@@ -217,6 +217,14 @@ packages:
       url: "https://pub.flutter-io.cn"
     source: hosted
     version: "3.1.2"
+  crop_your_image:
+    dependency: "direct main"
+    description:
+      name: crop_your_image
+      sha256: "14c8977b11a009dc5e73e0f6522970f93363e38183f1b2ffefe1676dc9c3f49d"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "2.0.0"
   cross_file:
     dependency: transitive
     description:
@@ -568,7 +576,7 @@ packages:
     source: hosted
     version: "4.1.2"
   image:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: image
       sha256: a1e7f4951e538a568e14b856702afc9ae1d2f4b202daced8d22c1b9cd211ce89