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/utils/url.ts

@@ -1,3 +1,5 @@
+import { isTauri } from '../mock-tauri'
+
 const URL_PATTERN = /^https?:\/\//i
 const BARE_DOMAIN_PATTERN = /^[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z]{2,})+([/?#]|$)/i
 
@@ -10,3 +12,13 @@ export function normalizeUrl(url: string): string {
   if (URL_PATTERN.test(url)) return url
   return `https://${url}`
 }
+
+/** Open a URL in the system browser. Uses Tauri opener plugin in native mode, window.open in browser. */
+export async function openExternalUrl(url: string): Promise<void> {
+  if (isTauri()) {
+    const { openUrl } = await import('@tauri-apps/plugin-opener')
+    await openUrl(url)
+  } else {
+    window.open(url, '_blank')
+  }
+}