fix: block notification permission dialogs via Chrome policy (#105)

## Summary
- Add `DefaultNotificationsSetting: 2` Chrome policy to automatically
deny all notification permission requests
- Prevents "site wants to show notifications" dialogs from appearing in
the automated browser environment

## Test plan
- [ ] Deploy updated chromium-headful image
- [ ] Navigate to a site that requests notification permissions (e.g.,
yahoo.com)
- [ ] Verify no notification permission dialog appears

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Sets Chrome policy `DefaultNotificationsSetting: 2` to automatically
deny all notification permission prompts.
> 
> - **Policies**:
>   - Update `shared/chromium-policies/managed/policy.json`:
> - Add `DefaultNotificationsSetting: 2` to auto-deny site notification
permission requests.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
94a7d12. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: rgarcia

shared/chromium-policies/managed/policy.json

@@ -1,5 +1,6 @@
 {
   "PasswordManagerEnabled": false,
   "AutofillCreditCardEnabled": false,
-  "TranslateEnabled": false
+  "TranslateEnabled": false,
+  "DefaultNotificationsSetting": 2
 }