feat: add svelte-sonner dependency and enhance NotificationBell component

- Added `svelte-sonner` to dependencies for improved notification handling.
- Refactored the `NotificationBell.svelte` component for better readability and maintainability, including code formatting and structure improvements.
- Updated `package.json` and `bun.lock` to reflect the new dependency.

bun.lock

@@ -8,6 +8,7 @@
         "chart.js": "^4.5.1",
         "lucide-svelte": "^0.548.0",
         "svelte-chartjs": "^3.1.5",
+        "svelte-sonner": "^1.0.5",
       },
       "devDependencies": {
         "@biomejs/biome": "^2.3.2",