fix(map): handle missing MapTiler key

[AyobamiH]

Problem

When NEXT_PUBLIC_MAPTILER_KEY is missing or blank, MapView still mounts Leaflet with a tile URL ending in ?key=. MapTiler rejects each tile request, but no React error is thrown. The existing error boundary therefore never replaces the gray, unusable map.

Closes #147.

Reproduction

On current main, with NEXT_PUBLIC_MAPTILER_KEY unset:

• /map responds successfully.
• The client constructs a MapTiler URL ending in ?key=.
• The tile endpoint responds with HTTP 403.
• No actionable state is shown to the user.

Changes

• Trim and validate the MapTiler key before mounting Leaflet.
• Render an accessible Basemap unavailable state when the key is missing or blank.
• Link the fallback to the self-hosting setup guide.
• Emit the configuration warning once in non-production environments.
• Document the visible symptom and recovery steps in SELF-HOSTING.md.
• Add a component regression test covering the fallback, setup link, and warning.

Why this layer

This is handled in MapView, not MapErrorBoundary, because failed HTTP tile requests do not throw a React rendering error. Stopping the map from mounting without its required configuration prevents the failing requests at their source.

Verification

• npm run lint
• npx tsc --noEmit
• npm run test:unit: 16 files and 129 tests passed
• env -u NEXT_PUBLIC_MAPTILER_KEY npx next build: production build passed with 489 generated pages

[vercel]

@AyobamiH is attempting to deploy a commit to the studentsuite Team on Vercel.

A member of the Team first needs to authorize it.

[AnayDhawan]

Thanks for the PR. If StudyMap has been useful, a star on the repo helps more students find it. Appreciated either way.

[AnayDhawan]

Merged, thanks. Nice work: the role="status" treatment, the dev-only warn with the module-level dedupe flag, and shipping a test alongside the fix all made this easy to review. The SELF-HOSTING.md note was a good call too.

Closes #147.