Fix/custom 404 page

[anushka11p]

📋 PR Checklist

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🔗 Related Issue

Closes #568

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📝 What does this PR do?

Adds a custom 404 (not-found) page for the Next.js App Router frontend. Previously, navigating to an unmatched route showed a generic/unclear error response instead of a branded page.

The new page:

• Clearly indicates the requested page doesn't exist
• Provides navigation options: Return to Home, and Go Back (browser history)
• Matches the existing app's styling (Tailwind + shadcn/ui Button component)

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Tested the affected API endpoints manually
• Added / updated tests

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

• The Button component in this project (@base-ui/react/button-based) doesn't support an asChild prop, so the navigation links use <Link><Button>...</Button></Link> composition instead.
• A pre-existing console warning from ThemeProvider/next-themes ("Encountered a script tag while rendering React component") appears on this route too — verified via git stash that it exists independently of this change, so it's out of scope here.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed