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/components/EnvironmentSwitcherButton.module.css

@@ -0,0 +1,44 @@
+.button {
+    display: inline-flex;
+    align-items: center;
+    gap: 8px;
+    padding: 6px 10px;
+    height: 32px;
+    background: var(--bg-surface);
+    border: 1px solid var(--border-subtle);
+    border-radius: 6px;
+    color: var(--text-primary);
+    font-size: 13px;
+    font-weight: 500;
+    cursor: pointer;
+    transition: background 0.15s ease, border-color 0.15s ease;
+}
+
+.button:hover {
+    background: var(--bg-surface-hover, var(--bg-surface));
+    border-color: var(--border-strong, var(--border-subtle));
+}
+
+.button:focus-visible {
+    outline: 2px solid var(--accent);
+    outline-offset: 2px;
+}
+
+.dot {
+    width: 10px;
+    height: 10px;
+    border-radius: 50%;
+    flex-shrink: 0;
+}
+
+.name {
+    max-width: 160px;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.chevron {
+    color: var(--text-muted);
+    flex-shrink: 0;
+}