[FEATURE]: Add Custom 404 Page (not-found.tsx)

[Atharva0506]

Feature and its Use Cases

Add custom 404 page using Next.js not-found.tsx.

Additional Context

Requirements

• Match UI/theme (Stable Viewpoints gradient aesthetics and layout standard)
• Responsive & Accessible (mobile scaling, aria clarity, keyboard focus states)
• Follow repo standards (Lucide icons, standard UI patterns, reused standard elements like Footer)
• Includes a clear, themed "Go to Homepage" CTA button and preserves the global header menu structure to prevent the user feeling disconnected from the rest of the app.

Code of Conduct

• I have joined the Discord server and will post updates there
• I have searched existing issues to avoid duplicates