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/styles/env-colors.css

@@ -0,0 +1,34 @@
+/*
+ * Per-environment preset color tokens. Used by:
+ *   - LayoutShell's 3px top-bar indicator
+ *   - EnvironmentSwitcherButton / EnvironmentSwitcherModal (color dot)
+ *   - EnvironmentsPage "Appearance" swatch grid
+ *
+ * Light-mode and dark-mode values are tuned for WCAG-AA contrast against the
+ * respective surface tokens (`--bg-surface`, `--text-primary`).
+ *
+ * Storage: plain lowercase string ("slate", "red", ...). Unknown values fall
+ * back to `slate` via `envColorVar()`.
+ */
+
+:root {
+    --env-color-slate:  #94a3b8;
+    --env-color-red:    #ef4444;
+    --env-color-amber:  #f59e0b;
+    --env-color-green:  #10b981;
+    --env-color-teal:   #14b8a6;
+    --env-color-blue:   #3b82f6;
+    --env-color-purple: #a855f7;
+    --env-color-pink:   #ec4899;
+}
+
+[data-theme='dark'] {
+    --env-color-slate:  #a1a9b8;
+    --env-color-red:    #f87171;
+    --env-color-amber:  #fbbf24;
+    --env-color-green:  #34d399;
+    --env-color-teal:   #2dd4bf;
+    --env-color-blue:   #60a5fa;
+    --env-color-purple: #c084fc;
+    --env-color-pink:   #f472b6;
+}