feat: Configure/Unconfigure toggle button, fix dropdown overflow

- Add Unconfigure() to IConfigWriter interface and both implementations
  (removes "unity" key from JSON config files)
- Button shows "Configure" (green) when not configured, "Unconfigure"
  (red) when already configured
- NeedsUpdate shows "Configure" to re-apply with current settings
- Fix client dropdown horizontal overflow with flexShrink=1, minWidth=0,
  overflow=hidden

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>