Add CM1 and CM2 boards, six backdrops and a quit button (0.8.0)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

data/exercises/cm2/cm2_tion_sion_qcm_07.json

@@ -0,0 +1,24 @@
+{
+  "id": "cm2_tion_sion_qcm_07",
+  "level": "CM2",
+  "domain": "orthographe",
+  "skill": "orthographe_cm2",
+  "rule_id": "regle_tion_sion_ssion",
+  "difficulty": 3,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon mot.",
+  "question": "La ... du volcan a duré trois jours.",
+  "answers": [
+    "érussion",
+    "éruction",
+    "éruption"
+  ],
+  "correct_answer": 2,
+  "hint": "Le son [sjɔ̃] s'écrit ici avec -ption.",
+  "explanation": "Le mot « éruption » s'écrit avec -ption.",
+  "review_tags": [
+    "orthographe_cm2",
+    "tion_sion"
+  ],
+  "review_status": "draft"
+}