fix: use Tauri opener plugin for URL property clicks

Replace window.open() with Tauri's opener plugin (openUrl) so that
clicking a URL property in the Properties panel opens the system
browser instead of failing silently or triggering edit mode.

- Install @tauri-apps/plugin-opener (npm + Cargo + capabilities)
- Add openExternalUrl() utility with Tauri/browser fallback
- Add stopPropagation to prevent accidental edit mode activation
- Add double-click guard (500ms debounce via ref)
- Update tests to mock openExternalUrl instead of window.open
- Add explicit test that URL click does not trigger onStartEdit

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

src-tauri/src/lib.rs

@@ -175,6 +175,7 @@ pub fn run() {
                 app.handle()
                     .plugin(tauri_plugin_updater::Builder::new().build())?;
                 app.handle().plugin(tauri_plugin_process::init())?;
+                app.handle().plugin(tauri_plugin_opener::init())?;
                 menu::setup_menu(app)?;
             }