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/api/queries/admin/environments.ts

@@ -9,6 +9,7 @@ export interface Environment {
   enabled: boolean;
   defaultContainerConfig: Record<string, unknown>;
   jarRetentionCount: number | null;
+  color: string;
   createdAt: string;
 }
 
@@ -22,6 +23,7 @@ export interface UpdateEnvironmentRequest {
   displayName: string;
   production: boolean;
   enabled: boolean;
+  color?: string;
 }
 
 export function useEnvironments() {