feat: add collections management and study coach features

- Implemented collection repository with CRUD operations for user collections.
- Created routes for managing collections, including listing, creating, updating, and deleting collections.
- Added sharing functionality for exams, allowing users to share and clone exams.
- Developed a study coach service that analyzes exam results and provides study tips based on weak areas.
- Introduced frontend components for managing collections, sharing exams, and displaying study coach insights.
- Enhanced user experience with loading states and error handling in shared components.

backend/src/routes/share.ts

@@ -0,0 +1,45 @@
+import { Router } from 'express'
+import { asyncHandler } from '../utils/asyncHandler.js'
+import { AppError } from '../middleware/error.js'
+import { requireAuth } from '../middleware/auth.js'
+import * as examRepo from '../repositories/examRepo.js'
+import type { Request } from 'express'
+
+const router = Router()
+
+// GET /api/share/:token — public, no auth required
+router.get(
+  '/:token',
+  asyncHandler(async (req: Request, res) => {
+    const token = String(req.params.token)
+    const exam = examRepo.getExamByShareToken(token)
+    if (!exam) {
+      throw new AppError(404, 'Shared exam not found')
+    }
+    const parsed = JSON.parse(exam.exam_json)
+    res.json({
+      title: exam.title,
+      subtitle: exam.subtitle,
+      questionCount: JSON.parse(exam.exam_json).questions?.length ?? 0,
+      exam: parsed,
+      shareMode: exam.share_mode ?? 'clone',
+    })
+  })
+)
+
+// POST /api/share/:token/clone — requires auth
+router.post(
+  '/:token/clone',
+  requireAuth,
+  asyncHandler(async (req: Request, res) => {
+    const token = String(req.params.token)
+    const source = examRepo.getExamByShareToken(token)
+    if (!source) {
+      throw new AppError(404, 'Shared exam not found')
+    }
+    const { id } = examRepo.cloneExam(req.user!.userId, source, token)
+    res.status(201).json({ id, title: source.title + ' (copy)' })
+  })
+)
+
+export default router