tauri-app: 8b click-through notifications

OS notification body already arrived as a 'maarcadetweet:notification'
DOM event (Phase 7b). The toast pill that surfaces the body is now
clickable: left-click navigates to the URL the notification was
about (at://<did>/<col>/<rkey> opens the thread, unknown URL
falls back to the home view); right-click dismisses without
navigating. tauri-plugin-notification v2.x does not expose a
reliable OS-level notification-click callback (it only shows the
notification), so the two-step pattern (OS click focuses the app
+ in-app toast click navigates) is the standard workaround.

lastNotificationUrl is now $state so the toast title updates
when a new notification arrives.

.vite/vitest/results.json

@@ -1 +1 @@
-{"version":"2.1.9","results":[[":crates/tauri-app/src/lib/api/client.test.ts",{"duration":31.70154099999999,"failed":false}],[":crates/tauri-app/src/lib/utils/localstorage.test.ts",{"duration":6.449833000000012,"failed":false}]]}
+{"version":"2.1.9","results":[[":crates/tauri-app/src/lib/api/client.test.ts",{"duration":31.23195799999999,"failed":false}],[":crates/tauri-app/src/lib/utils/localstorage.test.ts",{"duration":5.796832999999992,"failed":false}]]}