feat: implement OAuth login flow via core API

[TooAngel]

Summary

• Update useAuth to fetch OAuth URL from /api/auth/url and redirect to GitHub
• Add AuthCallback page to handle GitHub OAuth callback
• Update proxy to set httpOnly cookie on /api/auth/callback response

This implements the client-side of the OAuth flow that works with the core's new API endpoints (TooAngel/worlddriven#328).

Flow

1. User clicks "Login with GitHub"
2. Frontend calls GET /api/auth/url?redirect_uri=<callback> to get OAuth URL
3. Frontend redirects to GitHub OAuth
4. GitHub redirects back to /auth/callback?code=xxx
5. AuthCallback page calls POST /api/auth/callback with code
6. Proxy sets httpOnly cookie from response
7. User redirected to dashboard

Test plan

• Click login button redirects to GitHub OAuth
• After GitHub auth, user is redirected to dashboard
• Session cookie is set correctly
• Subsequent API calls are authenticated

[worlddriven]

🤖 Worlddriven Status

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🗓️ Merge Date: 2025-12-25 at 06:30:04 UTC (today)
📅 Started: 2025-12-23 at 06:30:04 UTC
⚡ Speed Factor: 0.20 (80% faster due to reviews)
✅ Positive votes: 4/5 contribution weight (coefficient: 0.80)
📈 Base Merge Time: 10 days → Current: 2 days

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📋 Recent Activity

• 2025-12-23, 06:30:12 - Pull request opened
• 2025-12-25, 06:51:34 - Pull request merged by worlddriven ✅

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This comment is automatically updated by worlddriven