Restore panel build in CI, add browser fallback for Open Admin Panel

- Re-add CGO panel builds with apt-get install of gcc/g++/mingw-w64/webkit2gtk
- Panel launch falls back to default browser if panel binary not found
- Restore panel binary in nfpm .deb package

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

packaging/nfpm.yaml

@@ -11,6 +11,8 @@ license: MIT
 contents:
   - src: ./syncwarden
     dst: /usr/bin/syncwarden
+  - src: ./syncwarden-panel
+    dst: /usr/bin/syncwarden-panel
   - src: ./packaging/linux/syncwarden.desktop
     dst: /etc/xdg/autostart/syncwarden.desktop
     type: config