feat(web): baseline accessibility semantics for the shells and login

[anabelle]

Refs #331. The baseline-semantics slice agreed in the issue: no live-region decisions here, no new dependencies.

What changes

• VisuallyHidden primitive in @facility/ui: the blessed way to give icon-only controls an accessible name. Tailwind's sr-only technique, never display: none, so content stays in the accessibility tree.
• SkipLink in both authenticated shells (org and project): first focusable element in the DOM, sr-only until focused, fixed top-left when visible, targets an explicit #main-content with tabIndex={-1} so focus actually moves and the next Tab lands inside the work area instead of back in the sidebar.
• Login page: <main> landmark, the brand mark becomes the <h1>, and title: "Sign in" rides the root template to render "Sign in · facility".

Tests

apps/web/test/a11y-baseline.test.tsx, deterministic and dependency-free: server-renders both real layouts with the API, topbar, and router-link modules mocked, then asserts the DOM contract (skip link precedes the sidebar nav, main carries id and tabindex, login has landmark, heading, and title; VisuallyHidden keeps text in the tree). Runs in the default node environment, no browser, no network, no credentials.

Verification

• pnpm --filter @facility/web test - 6 files, 37 tests passed (5 new)
• pnpm --filter @facility/web typecheck - clean
• pnpm --filter @facility/ui typecheck - clean
• pnpm exec biome check on changed files - clean after import-order fixes
• pnpm --filter @facility/web build - clean
• node guards/run.mjs - 2 guards ran, 0 failed

Not run: full pnpm verify (needs Docker-backed PostgreSQL; no API, database, or workspace boundary is touched) and the manual keyboard pass (recommended on merge: tab from the address bar on an org page, a project page, and the login page).

Effects

• User-visible: keyboard and screen-reader users can bypass the sidebar on every authenticated page; the login page exposes a landmark, a heading, and a descriptive title. feat on 0.x releases a patch.
• No persistence, migration, or compatibility effect. No security or money surface touched; no dependencies added.
• Follow-ups stay in Web UI accessibility: timeline has no live regions, no skip link, no sr-only primitive, no a11y guard in CI #331: the story-timeline announcement strategy (design note promised in the issue) and folding these surfaces into the test(web): fail on axe violations in ui fragments #332 guard once it lands.