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/flows.tsx

@@ -324,6 +324,7 @@ function FlowRow({
             }}
             className="rounded p-1.5 text-fg-subtle hover:bg-error/10 hover:text-error"
             title="Stop flow"
+            aria-label={`Stop flow ${flow.id}`}
           >
             <Square className="h-3.5 w-3.5" />
           </button>
@@ -451,7 +452,7 @@ export default function FlowsPage() {
       )}
 
       {error && (
-        <p className="mb-4 rounded-lg bg-error/10 px-4 py-2 text-sm text-error">
+        <p role="alert" className="mb-4 rounded-lg bg-error/10 px-4 py-2 text-sm text-error">
           {error}
         </p>
       )}