fix(router): redirect authenticated users on root

[dev-kp-eloper]

Fix/Feature: Redirect Authenticated Users on Root (Closes #17)

Description

This PR resolves the suboptimal UX flow where users who are already logged in are served the static marketing landing page when navigating to the root URL (/). It wraps the root route in a PublicOnlyRoute guard, which automatically redirects signed-in users straight to the dashboard (/dashboard).

Root Cause

In src/App.tsx, the root route (path="/") was mapped directly to <LandingPage /> without any authentication guards. Because it bypassed the route guards entirely, already-logged-in users had to manually navigate to /dashboard or /chats.

Changes

• src/App.tsx: Wrapped the root / path in the PublicOnlyRoute guard. This checks for active, unexpired JWT tokens in local storage synchronously prior to mounting LandingPage and performs an instant history replacement to /dashboard.

How to Verify

• npm run dev → navigate to http://localhost:5173/ while unauthenticated to confirm the Landing Page still renders perfectly.
• Simulate or complete a user login (storing a valid docchat_auth token in local storage).
• Navigate to the root route http://localhost:5173/ again and confirm it immediately redirects to /dashboard client-side without flashing the landing page first.
• Verify that clicking the browser back button does not redirect you again (clean history replacement).

Screenshots

Verified by client-side routing redirects.

[avishek0769]

Nice. Merging it after 1st June.