[FRONTEND] Add a 404 / not-found route with a styled page

[Trovic1]

Summary

Replace the silent catch-all redirect with a real, on-brand 404 page that offers a link home.

Scope

• frontend only

Likely files

• frontend/src/App.jsx
• frontend/src/pages/NotFound.jsx (new)

Acceptance criteria

• Unknown routes render a styled 404 using the design system
• Page links back to /dashboard (or /login if logged out)
• Uses the page-fade animation

Verification

• Visit a random URL and confirm the 404 renders

Labels: frontend, design, good first issue

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[codeurluce]

Hi! I'd like to work on this issue.