[app-bug] No accessibility audit — run axe-core and fix every page

[Gbangbolaoluwagbemiga]

Problem

Sanctifier has never had a structured a11y audit. Quick manual pass reveals: missing alt text on the home-page logo in some renderings, theme toggle button label that doesn't say what it does until you hover, no skip-to-content link, contrast issues in text-zinc-500 over white in light mode.

Acceptance Criteria

• Run @axe-core/react in dev and surface findings in console
• Run Lighthouse CI in CI on every PR; fail at <90 a11y score
• Fix every reported issue
• Add a "Skip to main content" link in <RootLayout> for screen-reader users

Pointers

• All routes under frontend/app/
• frontend/app/layout.tsx

Difficulty

Medium. Tooling is easy; the long tail of fixes adds up.