[FEATURE]: Add a dedicated 404 "Page Not Found" page for unknown routes

[RounakKumarAgarwal]

Feature and its Use Cases

Currently, OrgExplorer has no Not Found page. The catch-all route in src/App.jsx silently redirects every unknown path to the home page:

<Route path="*" element={} />

So if a user opens a mistyped or outdated link (e.g. /overveiw, /repos, or an old bookmarked route), they land on the home page with no indication that the page they wanted doesn't exist. This is confusing, especially for shared links, and makes broken links hard to notice.

Proposed feature: replace the redirect with a dedicated NotFoundPage that:

• Clearly shows a "404 – Page Not Found" message
• Displays the path the user attempted to visit
• Provides a "Go to Home" button (and optionally a "Go back" button)
• Renders inside the existing Layout (Navbar + Footer) so navigation stays available
• Follows the current theme and works in both light and dark mode
• Is responsive on mobile

Benefits:

• Users understand the link is broken instead of being silently redirected
• Easier to spot broken or outdated links shared across AOSSIE channels
• Standard UX pattern for single-page apps

Additional Context

• This is separate from the GitHub Pages reload 404 ([BUG]: Refreshing on any non-root route (e.g. /settings) returns GitHub Pages 404 #105, [BUG]: 404 error on reload #112, Reloading or directly opening any route returns a GitHub Pages 404 #202). That is already handled by the spaDeepLinkFallback() plugin in vite.config.js, which copies index.html to 404.html. This issue is only about unknown routes inside the app, after React Router loads.
• FEATURE : Add basic routing and separate 404 page (React Router) #40 originally asked for "basic routing and a separate 404 page". Routing exists now, but the separate 404 page does not; the catch-all currently redirects to /.
• Protected routes are unaffected: RequireAnalysis will continue redirecting to / when no analysis is available.
• Planned changes: add src/pages/NotFoundPage.jsx, update the * route in App.jsx, and add a small test (like RequireAnalysis.test.jsx) verifying that an unknown route renders the Not Found page.

@Zahnentferner approved working on this on Discord. I'd like to take it up, so could you please assign it to me? @rahul-vyas-dev

Code of Conduct

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