fix(login): prevent hydration redirect loop; show signed-in panel

[chundawat-h]

Summary
Fixes several login-related issues: hydration mismatch on the login page, unstable tab selection, and automatic redirect that prevented back-navigation. Also reverts unrelated UI/log changes introduced earlier.

Detailed changes

Convert login page to a client-only component and stabilize tab selection using a mount-only read of search params.
Remove the Suspense fallback that caused a server/client markup mismatch and hydration overlay.
Add an "already signed in" UI to avoid immediate redirect. This shows signed-in email and provides "Go to Dashboard" and "Sign Out" actions — preventing back-button loop.
Revert unrelated changes to supabaseClient and AuthContext log/error messages so we only touch login behavior.
Created a local .env file with provided credentials (not committed; it's gitignored).
Testing steps

Start frontend: npm run dev
Visit http://localhost:3000/login
Confirm page no longer shows hydration overlay and the login/signup tabs behave as expected.
If already signed-in, check that the panel appears and back navigation does not immediately redirect to dashboard.

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