Unknown routes render a blank page instead of a 404

[KonstantinMB]

Problem

Navigating to any URL the React router doesn't recognize returns HTTP 200 with an empty body instead of a 404 page. Users hitting a typo, stale link, or someone else's typo'd share end up looking at a blank cream-colored page with no nav, no message, no escape route.

Reproduction (verified via gstack QA)

$ B=~/.claude/skills/gstack/browse/dist/browse

$ $B goto https://exploreyc.com/this-is-not-a-real-page
Navigated to https://exploreyc.com/this-is-not-a-real-page (200)

$ $B wait --networkidle
$ $B js "document.body.innerHTML.length"
31    # i.e. essentially empty

$ $B js "document.body.textContent"
"\n    \n  \n\n"

Same happens for stale URLs that sound right but aren't actual routes — e.g. I found this while testing /validate-idea (the real route is /validator). A shared link to /validate-idea from a blog post, tweet, or old README would silently fail.

Proposed fix

Add a catch-all <Route path="*"> in the React router that renders a small 404 page with:

• A clear "Page not found" message
• The site nav (so the user can recover)
• A link back home
• Maybe a witty hacker-themed touch to match the rest of the design

In the future, also wire up vercel.json routes so the response status is 404 (not 200), but that's a follow-up — the React-level fix is the user-visible win.

Acceptance criteria

• https://exploreyc.com/whatever-junk renders the new 404 page with the site nav
• Visible "Page not found" text + link to home
• The fallback route is the last route in the React Router config (otherwise it eats everything)
• Page title is 404 Not Found — ExploreYC (works with the per-page-title issue once that lands)
• Bonus: log unknown-route hits to console with the bad path, for future analytics

Pointers

• frontend/src/App.tsx — where the <Routes> block lives. Add <Route path="*" element={<NotFoundPage />} /> at the end.
• Create frontend/src/pages/NotFoundPage.tsx for the 404 component.

Scope

Touches 2 files (1 new). Estimated time: 30 min - 1 hour.

Want to take this on? Comment below and I'll assign it.