feat(notifications): report completed background tasks

Register the Tauri notification plugin and request notification permission during startup. Send completion notifications for folder scans, embeddings, AI tagging, and duplicate scans, including failure counts where available.

package.json

@@ -8,7 +8,7 @@
     "build:vite": "tsc && vite build",
     "dev:app": "tauri dev",
     "dev:vite": "vite",
-"preview": "vite preview",
+    "preview": "vite preview",
     "tauri": "tauri"
   },
   "dependencies": {
@@ -16,6 +16,7 @@
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-dialog": "^2.7.0",
     "@tauri-apps/plugin-fs": "^2.5.0",
+    "@tauri-apps/plugin-notification": "^2.3.3",
     "@tauri-apps/plugin-opener": "^2",
     "d3-force": "^3.0.0",
     "framer-motion": "^12.38.0",