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>

.claude/rules/ui.md

@@ -25,6 +25,8 @@ The UI has 4 main tabs: **Exchanges**, **Dashboard**, **Runtime**, **Deployments
 - `ui/src/auth/auth-store.ts` — Zustand: accessToken, user, roles, login/logout
 - `ui/src/api/environment-store.ts` — Zustand: selected environment (localStorage)
 - `ui/src/components/ContentTabs.tsx` — main tab switcher
+- `ui/src/components/EnvironmentSwitcherButton.tsx` + `EnvironmentSwitcherModal.tsx` — explicit env picker (button in TopBar; DS `Modal`-based list). Replaces the retired `EnvironmentSelector` (All-Envs dropdown). When `envRecords.length > 0` and the stored `selectedEnv` no longer matches any env, `LayoutShell` opens the modal in `forced` mode (non-dismissible). Switcher pulls env records from `useEnvironments()` (admin endpoint; readable by VIEWER+).
+- `ui/src/components/env-colors.ts` + `ui/src/styles/env-colors.css` — 8-swatch preset palette for the per-environment color indicator. Tokens `--env-color-slate/red/amber/green/teal/blue/purple/pink` are defined for both light and dark themes. `envColorVar(name)` falls back to `slate` for unknown values. `LayoutShell` renders a 3px fixed top bar in the current env's color (z-index 900, below DS modals).
 - `ui/src/components/ExecutionDiagram/` — interactive trace view (canvas)
 - `ui/src/components/ProcessDiagram/` — ELK-rendered route diagram
 - `ui/src/hooks/useScope.ts` — TabKey type, scope inference