fix: replace stale /submissions?contributor= endpoint with GET /api/v1/me/submissions (Closes #387) [FaaFyfxR9WAQrL7FcAgEHJvztd8cVMxvjHRS55rw1nwH]

[waterWang]

What this fixes

The dashboard's /submissions?contributor= fetch was hitting a 404 because the backend has no /submissions route.

Changes

• Removed stale API_URL constant
• Added useAuth hook for auth token + API URL
• Replaced GET /submissions?contributor= with GET /api/v1/me/submissions (see feat(backend): add PostgreSQL persistence with TypeORM #38)
• Added Authorization: Bearer <token> header
• Added response normalization (handles { submissions: [...] } vs plain array)
• Updated useEffect dependency array

Closes #387