refact: remove exercise detail pages and modernize assessment interface

- Remove routes/exercises.py blueprint (only consultation routes)
- Delete templates/exercise_detail.html (intermediate page removed)
- Update app.py to remove exercises blueprint registration
- Modernize templates/assessment_detail.html with:
  * Hero section with gradient background
  * Action cards with hover effects and animations
  * Centered progress indicator with visual circles
  * Compact exercise structure display
  * Improved responsive design and UX

Part of Phase 2 UX improvements - eliminating intermediate pages for direct navigation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

routes/exercises.py

@@ -1,17 +0,0 @@
-from flask import Blueprint, render_template
-from models import Assessment, Exercise
-from utils import handle_db_errors
-
-bp = Blueprint('exercises', __name__)
-
-# Routes de consultation seulement - La création/modification se fait via le formulaire unifié d'évaluation
-
-@bp.route('/assessments/<int:assessment_id>/exercises/<int:id>')
-@handle_db_errors  
-def detail(assessment_id, id):
-    from sqlalchemy.orm import joinedload
-    assessment = Assessment.query.get_or_404(assessment_id)
-    exercise = Exercise.query.options(
-        joinedload(Exercise.grading_elements)
-    ).filter_by(id=id, assessment_id=assessment_id).first_or_404()
-    return render_template('exercise_detail.html', assessment=assessment, exercise=exercise)