Fix Linux build: add xdg-utils for AppImage bundling

The AppImage bundler requires xdg-open to be present.

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

.gitea/workflows/build-app.yml

@@ -35,7 +35,8 @@ jobs:
             build-essential \
             curl \
             wget \
-            file
+            file \
+            xdg-utils
 
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable