[umpireboulianne-design]

Summary

• Add a keyboard-focusable SkipLink component (apps/web/src/components/SkipLink.tsx) rendered as the first interactive element inside the App shell (apps/web/src/App.tsx) that moves keyboard focus and scroll position to #main-content without clobbering the hash router.
• Style .skip-link in apps/web/src/styles/base.css so it remains hidden off-screen by default and becomes visibly prominent on :focus / :focus-visible.
• Update LandingPage.tsx, DashboardPage.tsx, ArchitecturePage.tsx, and DemoPage.tsx so each page uses a <div> page wrapper, places primary navigation (<nav> / <aside className="case-nav">) first, and exposes a single unambiguous <main id="main-content" tabIndex={-1}> landmark for main content.
• Add automated tests in apps/web/src/skip-link-landmarks.test.tsx (imported by RoundStatusCard.test.tsx so pnpm web:test executes them in CI without altering package.json scripts) and document the landmark convention in ARCHITECTURE.md.

Supply-chain review

• No package scripts or dependencies changed. If changed, explain why below.
• No .vscode/, workflow, or other editor/config file changes. If changed, list them below.
• No lockfile changes. If changed, explain the update below.
• No binary or font asset changes. If changed, list and justify them below.
• I confirm no automatic task (such as preinstall/postinstall) was added.
• I confirm no hidden executable payload (encoded blob, obfuscated command, or unexpected binary) was added.
• Any changed executable or binary was reviewed by a maintainer.

Testing

• node --import tsx --test src/skip-link-landmarks.test.tsx in apps/web (6/6 tests pass)
• pnpm --filter @sub-rosa/web build (production Vite build succeeds)
• node scripts/check-direct-time-access.mjs (passes)
• node scripts/check-links.js (passes)

Closes #548
Fixes #548