feat(generation): add exercise instructions, grouped display, and wizard improvements

- Add per-section instructions field to exercise wizard with sensible defaults
- Group exercises by type with section headers showing instructions
- Pass type_instructions to backend AI prompt for context-aware generation
- Add instructions editing in exercise edit dialog
- Update Exercise interface to include instructions field

Made-with: Cursor

frontend/src/services/generation.service.ts

@@ -106,7 +106,7 @@ export const generationService = {
     });
   },
 
-  generateExercises(module: ExamModule, config: ExerciseConfig & { passage_text?: string }): Promise<{ questions: unknown[] }> {
+  generateExercises(module: ExamModule, config: ExerciseConfig & { passage_text?: string; type_counts?: Record<string, number>; difficulty?: string }): Promise<{ questions: unknown[] }> {
     return api.post(`/exam/${module}/generate`, {
       ...config,
       generate_exercises: true,