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/cm1/cm1_pluriel_aux_qcm_01.json

@@ -0,0 +1,24 @@
+{
+  "id": "cm1_pluriel_aux_qcm_01",
+  "level": "CM1",
+  "domain": "orthographe",
+  "skill": "orthographe_cm1",
+  "rule_id": "regle_pluriel_aux_eaux",
+  "difficulty": 1,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon mot.",
+  "question": "Les ... broutent dans le pré.",
+  "answers": [
+    "chevals",
+    "chevaux",
+    "chevaus"
+  ],
+  "correct_answer": 1,
+  "hint": "Le nom « cheval » finit par -al.",
+  "explanation": "Les noms en -al font -aux au pluriel : « chevaux ».",
+  "review_tags": [
+    "orthographe_cm1",
+    "pluriel_aux"
+  ],
+  "review_status": "draft"
+}