bug: no app/error.tsx or not-found.tsx — errors and 404s fall back to Next's default pages

[yakew7]

Problem

There is no error.tsx, not-found.tsx, or global-error.tsx anywhere under src/app:

$ find src/app -iname "error.tsx" -o -iname "not-found.tsx" -o -iname "global-error.tsx"
# (no output)

The app already has a custom PWA offline fallback (src/app/offline/page.tsx) and a custom map-error-boundary.tsx scoped to the map, but nothing for the App Router's own error/404 boundaries, so:

• An invalid route (or notFound() calls, e.g. src/app/city/[slug]/page.tsx and src/app/competitions/[slug]/page.tsx both call it for an unknown slug) falls back to Next.js's generic unstyled 404 page instead of the site's own design.

• A thrown error during rendering falls back to Next's generic unstyled error page. This is a real, reachable path: the three /docs/awesome-* pages (awesome-student-resources, awesome-study-resources, awesome-skills-plugins) call fetchAwesomeList() with no try/catch:

  $ grep -rn "fetchAwesomeList" "src/app/docs/(article)/awesome-skills-plugins/page.tsx" "src/app/docs/(article)/awesome-student-resources/page.tsx" "src/app/docs/(article)/awesome-study-resources/page.tsx"
  src/app/docs/(article)/awesome-skills-plugins/page.tsx:15:  const sections = await fetchAwesomeList("awesome-skills-plugins-for-students");
  src/app/docs/(article)/awesome-student-resources/page.tsx:15:  const sections = await fetchAwesomeList("awesome-student-resources");
  src/app/docs/(article)/awesome-study-resources/page.tsx:15:  const sections = await fetchAwesomeList("awesome-study-resources");
  

  and fetchAwesomeList() (src/lib/awesome-list.ts) throws whenever the GitHub raw-content fetch isn't res.ok (rate-limited, repo renamed, transient outage during the 24h ISR revalidation). Any of those makes the whole docs page crash to the default error screen.

Fix

Add src/app/error.tsx (client component, matches the site's theme/design) and src/app/not-found.tsx for a themed 404. Optionally wrap fetchAwesomeList() calls in a try/catch that renders a "couldn't load this list right now" EmptyState instead of crashing the page (the app already has an EmptyState component used elsewhere, e.g. results-list.tsx).

Acceptance criteria

• src/app/not-found.tsx renders a themed 404 (verify by visiting a nonexistent route)
• src/app/error.tsx renders a themed error screen
• At least one awesome-list page degrades gracefully instead of crashing when the upstream fetch fails