feat: wire resume upload and preferences to backend API

[Copilot]

Frontend pages were entirely disconnected from the backend — resume upload only did client-side text extraction, and preferences were saved with setTimeout mocks. This wires the full end-to-end flows.

New: frontend/src/lib/api.ts

Minimal typed API client shared across pages:

• Attaches Clerk ****** to every request via useAuth().getToken()
• Reads base URL from VITE_BACKEND_URL (falls back to http://localhost:3000)
• Exports: fetchResume, uploadResume, fetchPreferences, savePreferences, patchPreferences

ResumePage

After client-side text extraction, the file is now POSTed to POST /resume:

const token = await getToken();
await uploadResume(token, f); // multipart/form-data → backend → Cloudinary + DB + async AI parse

Status badge reflects upload state: PROCESSING → PARSED / FAILED.

ProfilePage

• On mount: GET /preferences and GET /resume fetched in parallel; form hydrated from server state instead of hardcoded defaults
• Save preferences: POST /preferences with { seniority, locationPreferences }; state updated from response
• Replace resume: POST /resume; returned fileUrl stored so the "View ↗" link in ResumeUpdateSection works
• Inline error display for both operations
• URL filename extraction uses new URL(fileUrl).pathname to handle query strings correctly