ui(env): explicit switcher button+modal, forced selection, 3px color bar

- Replace EnvironmentSelector "All Envs" dropdown with Button+Modal (DS Modal, forced on first-use).
- Add 8-swatch preset color picker in the Environment settings "Appearance" section; commits via useUpdateEnvironment.
- Render a 3px fixed top bar in the current env's color across every page (z-index 900, below DS modals).
- New env-colors tokens (--env-color-*, light + dark) and envColorVar() helper with slate fallback.
- Vitest coverage for button, modal, and color helpers (13 new specs).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/main.tsx

@@ -1,5 +1,6 @@
 import '@cameleer/design-system/style.css';
 import './index.css';
+import './styles/env-colors.css';
 import { StrictMode } from 'react';
 import { createRoot } from 'react-dom/client';
 import { RouterProvider } from 'react-router';