Add proper user authentication to prevent data loss

[natcrypto]

Problem

User identity is currently stored in localStorage (watchlist_user_id). This is fragile because:

• Browser can clear localStorage automatically (storage pressure, privacy settings)
• Users lose all their preferences if the key is lost
• No way to access data from another device
• Data becomes orphaned in the database with no way to recover

Current Flow

1. First visit → create anonymous user → store UUID in localStorage
2. Return visit → read UUID from localStorage → load user data
3. localStorage cleared → data loss (new user created, old data orphaned)

Proposed Solution

Add proper authentication with multiple options:

Option 1: Email-based auth (Recommended)

• Email + password registration
• "Magic link" passwordless login option
• Can migrate existing anonymous users by adding email to their account

Option 2: OAuth providers

• Google / GitHub / Apple sign-in
• Simpler UX but requires OAuth setup

Option 3: Device fingerprinting + email backup

• Keep anonymous users but prompt for email as backup
• Use email to recover if localStorage is lost

Implementation Notes

• Use existing auth service at auth.adaptiveedge.uk if possible
• Add migration path for existing anonymous users
• Consider adding "Link your account" prompt after onboarding

Acceptance Criteria

• Users can create account with email/password
• Users can log in from any device
• Existing anonymous users can link their data to an account
• Session persists across browser restarts
• Data is never lost due to localStorage clearing

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This issue was created after a user lost all their preference data due to localStorage being cleared