Migrate to TypeScript and Redux Toolkit

[Copilot]

Complete migration from JavaScript to TypeScript with strict mode enabled and Redux to Redux Toolkit across all domains.

Changes

TypeScript Migration

• Converted all 59 source files from .js/.jsx to .ts/.tsx
• Enforced strict TypeScript compilation (noImplicitAny, strictNullChecks, etc.)
• Added comprehensive type definitions for API responses, Redux state, and utilities
• Created typed Redux hooks (useAppSelector, useAppDispatch)

Redux Toolkit Refactor

• Migrated 6 domains to RTK slices with createSlice and createAsyncThunk:
  • app - search query, filters, terms
  • courses - course data and paginated search
  • instructors - instructor data and search
  • subjects - subject data and search
  • grades - course/instructor grade aggregations
  • explore - explore views
• Replaced createStore with configureStore
• Removed legacy action type enums and split action/reducer files (~1000 LOC)
• Co-located domain logic in slices

Type Safety

// Before: Untyped Redux usage
const data = useSelector(state => state.courses.data);
dispatch(fetchCourse(uuid));

// After: Fully typed with autocomplete
const data = useAppSelector(state => state.courses.data); // typed as Record<string, CourseData>
dispatch(fetchCourse(uuid)); // TypeScript validates uuid is string

Build Configuration

• Updated Vite config for TypeScript compilation
• Added type-check and lint npm scripts for CI integration
• Configured ESLint with TypeScript-specific rules

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