fix: make Run tabs keyboard accessible

[pppolf]

Summary

• give the Run workspace a complete WAI-ARIA tab/tablist/tabpanel relationship
• use a single roving tab stop and support ArrowLeft, ArrowRight, Home, and End navigation with automatic activation
• preserve DOM targets for inactive tab panels while rendering only the active panel's content
• localize the tablist accessible name
• add focused DOM regression tests for semantics, focus movement, wraparound, and activation

Why

The Run navigation previously assigned role="tab" to every button, but every tab remained in the page tab order and the tablist did not implement the keyboard interactions expected by assistive technology. The content also lacked a linked tabpanel, so tab-to-panel relationships were not exposed.

Validation

• Node.js 24.19.0
• npm ci (0 vulnerabilities)
• npm run test -- --run src/components/run-workspace-tabs.test.tsx (2/2)
• npm test (58 files, 220 tests)
• npm run typecheck
• npm run check:api
• npm run build
• npm audit --audit-level=high (0 vulnerabilities)
• git diff --check

The existing jsdom canvas warnings remain informational and unchanged.