fix: enhance notifications and validation features

- Added URL validation for notification providers to ensure only valid http/https URLs are accepted.
- Implemented tests for URL validation scenarios in the Notifications component.
- Updated translations for error messages related to invalid URLs in multiple languages.
- Introduced new hooks for managing security headers and access lists in tests.
- Enhanced the ProviderForm component to reset state correctly when switching between add and edit modes.
- Improved user feedback with update indicators after saving changes to notification providers.
- Added mock implementations for new hooks in various test files to ensure consistent testing behavior.

frontend/src/pages/__tests__/ProxyHosts-coverage.test.tsx

@@ -33,6 +33,9 @@ vi.mock('../../api/accessLists', () => ({ accessListsApi: { list: vi.fn() } }))
 vi.mock('../../api/settings', () => ({ getSettings: vi.fn() }))
 vi.mock('../../api/backups', () => ({ createBackup: vi.fn() }))
 vi.mock('../../api/uptime', () => ({ getMonitors: vi.fn() }))
+vi.mock('../../hooks/useSecurityHeaders', () => ({
+  useSecurityHeaderProfiles: vi.fn(() => ({ data: [], isLoading: false, error: null })),
+}))
 
 const createQueryClient = () => new QueryClient({ defaultOptions: { queries: { retry: false, gcTime: 0 }, mutations: { retry: false } } })