fix(web): add baseline accessibility landmarks and skip navigation

[ALVZ93]

Refs #331.

What changes

• adds an exported VisuallyHidden primitive to @facility/ui
• adds a keyboard-first skip link to both authenticated app shells
• gives both skip-link targets a shared main-content id and tabIndex={-1} so the destination can receive focus reliably
• gives the login page a <main> landmark, a real <h1> with visually hidden page-purpose text, and a descriptive Sign in · facility document title
• adds deterministic renderToStaticMarkup regression coverage for the primitive, skip-link target, and login semantics

Scope

This intentionally leaves the story-timeline announcement strategy for a separate follow-up. It also does not add axe coverage because #332 is already handling the automated accessibility guard.

Verification

• branch is based directly on current main (c14bc6d)
• no dependencies or lockfile changes
• no API, database, security, or money-sensitive surface touched
• focused tests are included; repository CI is expected to run the full project checks on this PR