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>

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+Alice Johnson is a senior engineer at Acme Corporation. Acme develops CloudSync, a cloud storage platform. CloudSync uses Amazon Web Services for hosting.

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+Bob Chen is the CTO of Acme Corporation. Alice reports to Bob. CloudSync was launched in 2024 and competes with Dropbox.

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+Alice Johnson is a senior engineer at Acme Corporation. Acme develops CloudSync, a cloud storage platform. CloudSync uses Amazon Web Services for hosting.

ts/data/librarian/docs/b7bc3c30-65fc-424b-bb8b-6dfeca739250.bin

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+Bob Chen is the CTO of Acme Corporation. Alice reports to Bob. CloudSync was launched in 2024 and competes with Dropbox.

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+The Eiffel Tower is a wrought-iron lattice tower in Paris, France