fix: comprehensive a11y and contrast QA pass across workbench

Automated QA loop (6 parallel browser agents, 2 rounds) found and fixed
15 accessibility, contrast, and responsive issues across all 8 pages:

- WCAG contrast: light-mode warning (#854d0e), error (#b91c1c), toggle
  off-state (surface-400), connection badge (fg-muted)
- ARIA: mode selector group+pressed, tab pattern ids+labelledby, nav
  and aside labels, dialog focus-return, alert roles on banners
- Responsive: library header flex-wrap, search/button aria-labels
- Focus: NavLink visible ring, dialog close button ring

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ts/packages/workbench/src/pages/graph.tsx

@@ -500,6 +500,7 @@ export default function GraphPage() {
               value={searchTerm}
               onChange={(e) => setSearchTerm(e.target.value)}
               placeholder="Search nodes..."
+              aria-label="Search nodes"
               className="w-48 rounded-lg border border-border bg-surface-100 py-1.5 pl-8 pr-3 text-xs text-fg placeholder:text-fg-subtle focus:border-brand-500 focus:outline-none focus:ring-1 focus:ring-brand-500"
             />
             {searchTerm && (