feat: redesign MCP settings with test-before-add UX

- Add testCommand endpoint to test a command before saving to config
- Separate Test and Add buttons in the form
- Test shows success with tool count and names, or error message
- Polished UI with better spacing, labels, and visual hierarchy
- Server list shows tool badge when tested successfully
- Error messages use styled alert boxes
- Empty state with icon for no projects

Author: ammar-agent