chore: Enhance backend test coverage and add new functional tests for Security page

- Added tests to `proxyhost_service_validation_test.go` to validate fallback parsing and handle invalid hostname characters.
- Introduced new tests for DNS challenge validation in `proxyhost_service_validation_test.go`.
- Updated `current_spec.md` to reflect changes in testing strategy and coverage goals for PR #729.
- Enhanced `Security.functional.test.tsx` to include navigation test for Notifications button.
- Mocked `useNavigate` from `react-router-dom` to verify navigation behavior in Security page tests.

.github/agents/Frontend_Dev.agent.md

@@ -48,8 +48,7 @@ You are a SENIOR REACT/TYPESCRIPT ENGINEER with deep expertise in:
    - Run tests with `npm test` in `frontend/` directory
 
 4. **Quality Checks**:
-   - Run `npm run lint` to check for linting issues
-   - Run `npm run typecheck` for TypeScript errors
+   - Run `pre-commit run --all-files` to ensure linting and formatting
    - Ensure accessibility with proper ARIA attributes
 </workflow>