Remove auth redirect and add sign-in button to sidebar footer (#4)
### TL;DR
Allow unauthenticated users to access the chats route, showing a "Sign In" button in the sidebar footer instead of redirecting them away.
### What changed?
The `/chats` route no longer redirects unauthenticated users to the sign-in page. Instead, a "Sign In" button is displayed in the sidebar footer when the user is signed out, using Clerk's `SignedOut` and `SignInButton` components. The `SignedIn` guard around the user button remains intact.
### How to test?
1. Open the app while signed out and navigate to `/chats`.
2. Verify you are no longer redirected to the sign-in page.
3. Confirm the sidebar footer displays a "Sign In" button.
4. Click the "Sign In" button and verify it triggers the Clerk sign-in flow.
5. Sign in and confirm the sidebar footer switches to showing the user button.
### Why make this change?
Redirecting unauthenticated users away from the chats route creates a poor experience for users who may want to explore the app before signing in. Surfacing a "Sign In" prompt directly in the sidebar is a softer, more welcoming approach that keeps users in context.
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