fix: add cold start loading banner for Render free-tier delay

[rachel-d-07]

• Added slowLoad state that triggers after 3 seconds of active API call
• Shows a friendly warning message when backend takes long to respond
• Banner auto-clears when loading completes
• Targets both email/password login and Google login flows

Closes #1462

📌 Description

Added a cold start warning banner to Login.jsx that appears after 3 seconds
of loading. This gives users feedback when the Render free-tier backend is
waking up, instead of showing a blank screen.

🔗 Related Issue

Closes #1462

🛠 Changes Made

• Added slowLoad state to track slow API responses
• Added useEffect timer that triggers slowLoad after 3 seconds of active loading
• Added warning banner that shows when slowLoad is true during login
• Banner auto-clears when loading completes or fails
• Works for both email/password and Google login flows

📸 Screenshots (if applicable)

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue

🚀 Notes for Reviewers

Anything specific you want reviewed.
The banner only appears when isSubmitLoading or isGoogleLoading is true
for more than 3 seconds — it will not show during normal fast responses.

[aryandas2911]

linting error