ci: install Linux dev dependencies for wails3 CLI build

The wails3 CLI requires GTK3/WebKit2GTK dev packages to build on
the GitHub runner (for CGO).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/build-cross-image.yml

@@ -124,6 +124,11 @@ jobs:
         with:
           node-version: '20'
 
+      - name: Install Linux dev dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libgtk-3-dev libwebkit2gtk-4.1-dev
+
       - name: Install wails3 CLI
         run: |
           cd v3