Add React/TypeScript instructor dashboard alongside existing KnockoutJS frontend

[Copilot]

Adds a standalone React + TypeScript frontend (Vite) that coexists with the existing KnockoutJS frontend, plus an instructor dashboard page visualizing submission metrics from the submission_counts, course_counts, assignment_counts, and user_counts tables.

New: frontend-react/

• src/types/models.ts — TypeScript types mirroring all backend models: Submission, Course, Assignment, CourseCounts, UserCounts, and all SubmissionMetrics enum values
• src/api/client.ts — typed fetch wrappers for every backend route group (/api/*, /courses/*, /assignments/*, /grading/*, /blockpy/*); reads window.$URL_ROOT injected by Flask
• src/api/dashboardUtils.ts — aggregates raw fake_dashboard rows into per-assignment and per-student summaries (error rates, assertion success rate, avg edit/intervention time)
• src/hooks/useFetch.ts — useCourses / useCourseMetrics hooks with loading/error/refetch state
• src/pages/InstructorDashboard.tsx — dashboard with course selector, stat cards, tabbed charts (Recharts) and tables, CSV export
• src/components/dashboard/ — StatCard, AssignmentSubmissionsChart, ErrorRateChart, MetricsTable, StudentTable

Flask integration

• GET /dashboard route added to basic.py (@login_required)
• templates/react/dashboard.html — extends existing layout; injects $URL_ROOT, accessToken, currentUserId for the API client
• "Dashboard" nav link added to shared navbar for authenticated users
• Vite builds to static/libs/blockpy_react/ with stable (non-hashed) output filenames; directory gitignored

Dashboard capabilities

Consumes /courses/fake_dashboard?mode=json (existing endpoint):

• Stat cards: submission count, unique students/assignments, avg time, avg score, overall error rates
• Charts tab: bar charts for submission counts + avg edits, and syntax/runtime error + assertion success rates per assignment
• Assignment table: color-coded per-assignment breakdown of all SubmissionMetrics
• Student table: per-student score, time, edits, correct count
• CSV export: proxies to ?mode=csv