Address code review comments for authentication system

[Copilot]

📝 Description

Addresses all code review feedback from the automated review. Key improvements:

Backend (auth.py, supabase.py)

• Generic error messages to avoid leaking Supabase internals
• Extracted hash_email() utility for reuse
• Preserved exception chains with from db_error
• Fixed misleading docstring re: service role key vs anon key

Frontend

• API_ENDPOINT extracted to module constant (App.jsx)
• Consolidated duplicate message listeners (background.js)
• SUCCESS_MESSAGE_TIMEOUT named constant (LoginPage.jsx)
• Added aria-live accessibility attributes (confirm-email.html)
• Added standard background-clip alongside -webkit- prefix

Config

• Removed HTTPS localhost redirect URL causing cert issues

# Before: Raw error exposed
raise HTTPException(status_code=409, detail=msg)

# After: Generic message, details logged
logger.exception(f"Supabase sign_up failed: {msg}")
raise HTTPException(status_code=409, detail="This email is already registered")

🎯 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔨 Refactoring (no functional changes)
• 🧪 Tests (adding or updating tests)
• 🔧 Chore (dependency updates, config changes, etc.)

🧪 Testing

• I have tested this change locally
• I have added/updated tests for this change
• All existing tests pass

📋 Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

📸 Screenshots (if applicable)

N/A - No UI changes

🔗 Related Issues

Follow-up to PR #45 code review comments

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