feat(frontend): add 404 catch-all route and NotFoundPage component

[fadeelah2007]

Closes #144

Summary

Adds a custom 404 Not Found page component (NotFoundPage.tsx) and configures catch-all wildcard routes in App.tsx so users navigating to undefined URLs receive clear feedback and navigation options instead of a blank screen inside AppShell.

Changes

• Created frontend/src/pages/NotFoundPage.tsx with a primary "404 — Page Not Found" heading, brief explanation, and styled action links ("Go to Dashboard" and "Go Home").
• Used project CSS variables (var(--text-primary), var(--text-secondary), var(--bg-primary), etc.) for consistent dark theme styling.
• Updated frontend/src/App.tsx with wildcard <Route path="*" element={<NotFoundPage />} /> both inside AppShell (for app sub-routes) and outside (for root-level unknown routes).

Testing

• Verified navigation to undefined routes (e.g., /random) renders the styled 404 page within the AppShell layout.
• Tested "Go to Dashboard" button navigation to /dashboard.
• Tested "Go Home" button navigation to /.
• Confirmed responsive layout across desktop and mobile viewports.

Related Issue

#144

Checklist

• Tests pass
• Lint is clean
• Documentation updated if needed

[vercel]

@fadeelah2007 is attempting to deploy a commit to the Jaja's projects Team on Vercel.

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[devJaja]

Well Done @fadeelah2007

I can see all The CI checks passed.

LGTM