feat: add catch-all 404 NotFound route

[IfyJustin91]

Summary

Implements issue #19 — adds a catch-all path="*" route so any unknown URL renders a proper 404 page instead of a blank area.

Changes

• src/pages/NotFound.tsx (new) — 404 page with:
  • Large 404 status indicator
  • "Page not found" heading
  • The unrecognised path displayed in a code block
  • Quick-nav links back to Home, Search, Graph, XDR Inspector, and Sandbox
  • A "← Go back" button
• src/App.tsx — lazy-imports NotFound and registers <Route path="*" element={<NotFound />} /> as the last route, inside the existing ErrorBoundary/Suspense layout
• test/NotFound.test.tsx (new) — 8 tests covering:
  • Unknown path renders the 404 content
  • Unrecognised path is shown in the message
  • "Page not found" heading is present
  • At least one working link back to /
  • All five quick-nav links rendered
  • Go-back button rendered
  • Known route /search is not swallowed by the catch-all
  • Root path / is not swallowed by the catch-all

Testing

npm test

All 8 new tests pass. TypeScript compiles clean (tsc --noEmit).

Closes #19

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