feat(settings): build notification preferences page with validation and persistence

[SharifIbrahimDev]

Summary

This PR implements the Notification Preferences Page inside apps/web/app/settings/notifications/, enabling users to configure alert delivery preferences across Discord, Telegram, and Webhook channels.

Closes #105

Key Changes

1. Created Notification Preferences Page (page.tsx):
   • Toggles to enable/disable Discord, Telegram, and Webhook alerts.
   • Form fields for Discord (Webhook URL), Telegram (Bot Token, Chat ID), and Custom Webhooks (Endpoint URL, Signing Secret).
   • Client-side validation:
     • Discord Webhook URL matches standard Discord api webhook structures.
     • Telegram Bot Token matches \d+:[A-Za-z0-9_-]{35} format.
     • Telegram Chat ID is a valid integer.
     • Webhook URL has a valid URL format with HTTP/HTTPS protocols.
     • Webhook Signing Secret is non-empty and at least 8 characters.
   • LocalStorage persistence saving preferences and showing success alerts or input-level error messages.
2. Custom Styles (notifications.css):
   • Modern cards, toggles, success status boxes, and input styles consistent with the overall Sentinel design system.
3. Navigation Integration (SettingsNav.tsx):
   • Added a link to the Notifications preferences page.
4. Comprehensive Unit Tests (page.spec.tsx):
   • Asserts initial state, interactive toggling visibility, form submission errors on invalid input, and successful persistence to localStorage.

Verification

• Linting & Formatting: Verified clean run with npm run format:check and npm run lint specifically on modified files.
• Unit Tests: Executed all frontend tests with npm run test:dashboard. All 10 suites (94 test cases) passed successfully.

[mijinummi]

LGTM