feat(study): replace mock data with real backend service

[MuenYu]

Summary

Replaced the hardcoded mock vocabulary list in StudyView.tsx with a real backend service that fetches flashcards from the database and persists review results with spaced repetition scheduling.

Changes

New Files

• lib/study/service.ts: Backend service with:

  • getStudyQueueForUser() - Fetches "pending + due" flashcards (state != suspended)
  • submitReviewForUser() - Records reviews and applies SM-2 scheduling logic
  • getStudyStatsForUser() - Helper for due count/today's reviews

• app/api/study/queue/route.ts: API endpoint to fetch study cards

• app/api/study/review/route.ts: API endpoint to submit review results

Modified Files

• app/(dashboard)/study/StudyView.tsx:
  • Removed MOCK_VOCAB_LIST hardcoded data
  • Added API integration for fetching cards on mount
  • Implemented infinite queue behavior (auto-refills when remaining <= 5)
  • Added empty state UI when no cards are due
  • Persists all review data to the database

Behavior

Feature	Implementation
Session content	"Pending + due" cards (state != suspended)
Validation	Kept local heuristic (10+ chars = pass)
Session length	Infinite queue (fetches more as you complete)
SRS	Proper DB writes to flashcards (dueAt, state, easeFactor, streak, etc.)

Testing

bun db:seed   # if DB not seeded
bun dev       # starts dev server at localhost:3000
# Visit /study to see real cards from database

Type Checking

✅ Passes TypeScript: npx tsc --noEmit