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-isolated.test.tsx

@@ -79,6 +79,10 @@ describe('ProxyHosts page - coverage targets (isolated)', () => {
 
     vi.doMock('../../hooks/useAccessLists', () => ({ useAccessLists: vi.fn(() => ({ data: [] })) }))
 
+    vi.doMock('../../hooks/useSecurityHeaders', () => ({
+      useSecurityHeaderProfiles: vi.fn(() => ({ data: [], isLoading: false, error: null }))
+    }))
+
     vi.doMock('../../api/settings', () => ({ getSettings: vi.fn(() => Promise.resolve({ 'ui.domain_link_behavior': 'new_window' })) }))
 
     // Import page after mocks are in place