fix: preserve session on server cold starts and auto-redirect authenticated users to dashboard

[Atanu2k4]

Summary

Fixes an issue where users were unexpectedly logged out when the server experienced a restart or cold start. It also ensures that authenticated users arriving at the root landing page (/) are automatically redirected to their dashboard rather than being prompted to log in again.

Related issue

Fixes #68

Changes

• AuthContext.tsx: Restricted session clearing to definitive 401 or 404 responses. Network issues and transient server errors (500 range) will now gracefully fall back to the cached user session instead of wiping localStorage.
• page.tsx: Added validation checks for the active user state on the landing page, automatically replacing the route with /dashboard if an active session is detected.

Testing

• Redirection verification: Navigated directly to the root domain (/) with a valid token in localStorage and verified immediate redirection to /dashboard.
• Cold start resiliency testing: Simulated a failed/slow API endpoint and confirmed that network failures no longer destroy the local credentials cache.

Checklist

• Title follows the repository PR format
• The issue is linked with Fixes #68
• Testing notes are included

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Fix: Session logout on server cold-starts & missing auto-redirect on home page #68)