feat: implement branded custom 404 page with dashboard redirect

[Smrithi-krishna]

Summary

This PR creates a custom, branded not-found.tsx page for the Next.js App Router, replacing the generic Next.js 404 screen. It features the DevTrack logo and typography, supports dark/light mode via existing global CSS custom properties, and includes a call-to-action button to redirect users back to safety.

Closes #971

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

-Created src/app/not-found.tsx to catch all invalid application routes globally.

• Implemented responsive Tailwind CSS layout featuring the DevTrack logo icon and custom 404 branding.
• Mapped all text and background elements to CSS custom variables (var(--primary), var(--foreground), var(--muted-foreground)) to ensure native compliance with light and dark mode toggles.
• Added a primary navigation CTA button using next/link to redirect users back to the dashboard/home page (/).

How to Test

Steps for the reviewer to verify this works:

1. Ensure your local environment variables are set up (e.g., mock keys in .env.local to bypass the src/lib/supabase.ts safety guards if testing fresh).
2. Run the development server with npm run dev.
3. Navigate to a non-existent URL path, such as http://localhost:3000/invalid-test-route-404.
4. Verify that the custom DevTrack 404 UI renders correctly, fits within the global layout, and that the "Go to Dashboard" button correctly routes you back to /.

Screenshots (if UI change)

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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[Smrithi-krishna]

hi @Priyanshu-byte-coder can you please check this