Fix app shell keyboard accessibility

[simple-agent-manager]

status: ready-for-review
local_checks:

• pnpm --filter @simple-agent-manager/web test -- AppShell.test.tsx
• pnpm --filter @simple-agent-manager/web lint
• pnpm --filter @simple-agent-manager/web typecheck
• pnpm --filter @simple-agent-manager/web build
  review_evidence:
• UI review task 01KYC75SZCAMN4ZTH742E5AZR4 flagged missing skip-to-content and weak focus visibility as critical/high accessibility findings.
• Scope intentionally limited to AppShell skip link, main target, global keyboard focus fallback, and a focused AppShell regression test.
• No routing, state-management, API, auth, or workflow behavior changed.

Summary

• add a keyboard-accessible skip-to-content link in both mobile and desktop AppShell layouts
• add stable #main-content targets without changing rendered page content or routing
• add a global :focus-visible fallback ring for keyboard users
• cover the AppShell accessibility contract in unit tests

Validation

• pnpm --filter @simple-agent-manager/web test -- AppShell.test.tsx
• pnpm --filter @simple-agent-manager/web lint (passes with pre-existing warnings)
• pnpm --filter @simple-agent-manager/web typecheck
• pnpm --filter @simple-agent-manager/web build

Notes

This PR is intentionally narrow and non-breaking. It does not address broader UI review items like route-level code splitting or TanStack Query migration.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

---

_{Comparing sam/ui-accessibility-review-fix (cbad5b5) with main (44adc7e)}

[simple-agent-manager]

Closing as superseded by the delegated accessibility remediation PR #1674, which has stronger scoped tests and should be the canonical PR for this finding. No code from this duplicate PR should be merged separately.