feat: add custom 404 page

[WorldsBestSoftwareDeveloper]

Summary

This pull request adds a custom not-found.tsx page for the frontend application.

Changes Made

• Added a branded custom 404 page under frontend/app/not-found.tsx
• Reused the existing JointSave logo, UI components, and visual styling
• Reused the existing Button and ThemeToggle components
• Supports both light and dark mode
• Displays a clear "Page Not Found" message
• Routes connected users to /dashboard
• Routes users who are not connected to the landing page (/)

Verification

• Tested by visiting a non-existent route locally
• Confirmed the custom 404 page renders correctly
• Verified the navigation button works as expected
• No browser console errors observed

Closes #127

[Sendi0011]

Gm @WorldsBestSoftwareDeveloper I Really like the 404 page — the dashboard-vs-landing-page logic based on wallet connection is a nice touch. This branch has fallen a bit behind main though, enough that merging now would accidentally remove a few things that shipped since you started (changelog, sitemap/robots, the My Groups search). Could you pull the latest main in before we merge? Once that's done this should be good to go.

[WorldsBestSoftwareDeveloper]

Gm @Sendi0011, thank you for the feedback! I've merged the latest upstream/main into my branch and pushed the updated changes. The branch is now up to date with main. Please let me know if there's anything else you'd like me to adjust.

[Sendi0011]

This is good👍