feat: add branded not found route

[Maximum-Prosper]

Closes #348

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Summary

Adds a branded catch-all NotFound route for unknown paths so users are guided back to the Access Layer marketplace instead of landing on a blank or broken page.

What changed

• Added NotFoundPage with Access Layer branding and marketplace-focused copy
• Wired the router to render the new page for unmatched routes
• Added a test to verify the fallback page renders and the CTA points back to /

Verification

• Added unit coverage for the new route page
• Local lint/build verification could not be run in this environment because the available Node toolchain was blocked here