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_08.json

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+{
+  "id": "cm1_pluriel_aux_qcm_08",
+  "level": "CM1",
+  "domain": "orthographe",
+  "skill": "orthographe_cm1",
+  "rule_id": "regle_pluriel_aux_eaux",
+  "difficulty": 3,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon mot.",
+  "question": "Deux ... se promènent dans la forêt.",
+  "answers": [
+    "oiseau",
+    "oiseaus",
+    "oiseaux"
+  ],
+  "correct_answer": 2,
+  "hint": "Le nom « oiseau » finit par -eau.",
+  "explanation": "Les noms en -eau prennent un -x au pluriel : « oiseaux ».",
+  "review_tags": [
+    "orthographe_cm1",
+    "pluriel_aux"
+  ],
+  "review_status": "draft"
+}