Rework AI-Typewriter en application d'arrière-plan multi-profils

- Rollback gestion d'équations : écriture littérale caractère par caractère
  (suppression de math_format.py et de la normalisation Unicode).
- Nouveau profil par défaut 'Mathématiques (LaTeX)' : le modèle émet du
  LaTeX encadré [EQ]...[/EQ], intercepté par KeyStepper qui déclenche
  Alt+= en début d'équation et -> en fin.
- Config multi-profils dans %APPDATA%/ai-typewriter/config.json
  (ConfigStore + profils par défaut auto-créés).
- Icône de zone de notification (pystray) : Ouvrir les logs (temps réel),
  Ajouter/Modifier un profil, Gérer l'authentification, Quitter.
- Sélecteur de modèles : modèles locaux Ollama + recherche/téléchargement
  depuis la bibliothèque publique ; fournisseurs tiers (OpenAI, OpenRouter,
  Gemini, custom).
- Clés d'API stockées de façon sécurisée dans le Gestionnaire d'identifiants
  Windows via keyring (jamais en clair dans config.json).
- Tests : 44 tests verts (stepper, profils, clients IA, credentials,
  catalogue de modèles, moteur).

tests/test_engine.py

@@ -0,0 +1,106 @@
+"""Tests du moteur : capture -> IA -> dactylographie (tout mocké)."""
+
+from types import SimpleNamespace
+
+from ai_typewriter.config import ConfigStore
+from ai_typewriter.engine import AITypewriterEngine
+
+
+def _make_store(tmp_path) -> ConfigStore:
+    store = ConfigStore(tmp_path / "config.json")
+    store.ensure_defaults()
+    store.load()
+    return store
+
+
+def _fake_keyboard(monkeypatch, typed, sent):
+    hook = {}
+    monkeypatch.setattr(
+        "ai_typewriter.key_stepper.keyboard.write",
+        lambda char, delay=0, exact=True: typed.append(char),
+    )
+    monkeypatch.setattr("ai_typewriter.key_stepper.keyboard.send", lambda v: sent.append(v))
+    monkeypatch.setattr(
+        "ai_typewriter.key_stepper.keyboard.hook",
+        lambda callback, suppress=True: hook.update({"callback": callback}) or "hook",
+    )
+    monkeypatch.setattr("ai_typewriter.key_stepper.keyboard.unhook", lambda v: None)
+    return hook
+
+
+def test_capture_ask_and_step_types_response(monkeypatch, tmp_path):
+    store = _make_store(tmp_path)
+    engine = AITypewriterEngine(store)
+    typed, sent = [], []
+    hook = _fake_keyboard(monkeypatch, typed, sent)
+
+    monkeypatch.setattr(
+        "ai_typewriter.engine.capture_clipboard", lambda: "Question de test"
+    )
+    monkeypatch.setattr(
+        "ai_typewriter.engine.ask_ai", lambda prompt, profile, store: "Réponse IA"
+    )
+
+    engine.capture_ask_and_step()
+    assert engine._stepper is not None
+    hook["callback"](SimpleNamespace(event_type="down"))
+    hook["callback"](SimpleNamespace(event_type="down"))
+    assert typed == ["R", "é"]
+
+
+def test_capture_ask_and_step_with_equations(monkeypatch, tmp_path):
+    store = _make_store(tmp_path)
+    store.set_active("Mathématiques (LaTeX)")
+    engine = AITypewriterEngine(store)
+    typed, sent = [], []
+    hook = _fake_keyboard(monkeypatch, typed, sent)
+
+    monkeypatch.setattr(
+        "ai_typewriter.engine.capture_clipboard", lambda: "Calcule"
+    )
+    monkeypatch.setattr(
+        "ai_typewriter.engine.ask_ai", lambda prompt, profile, store: r"[EQ]a=x[/EQ]"
+    )
+
+    engine.capture_ask_and_step()
+    # 1:[EQ]->alt+= ; a: char ; =: char ; x: char ; [/EQ]->right  => 5 actions
+    for _ in range(5):
+        hook["callback"](SimpleNamespace(event_type="down"))
+
+    assert sent == ["alt+=", "right"]
+    assert typed == ["a", "=", "x"]
+
+
+def test_empty_clipboard_does_not_ask(monkeypatch, tmp_path):
+    store = _make_store(tmp_path)
+    engine = AITypewriterEngine(store)
+    called = []
+    monkeypatch.setattr("ai_typewriter.engine.capture_clipboard", lambda: "   ")
+    monkeypatch.setattr(
+        "ai_typewriter.engine.ask_ai",
+        lambda prompt, profile, store: called.append(prompt) or "x",
+    )
+    engine.capture_ask_and_step()
+    assert called == []
+
+
+def test_ask_only_returns_answer(monkeypatch, tmp_path):
+    store = _make_store(tmp_path)
+    engine = AITypewriterEngine(store)
+    monkeypatch.setattr(
+        "ai_typewriter.engine.ask_ai",
+        lambda prompt, profile, store: "reponse-brute",
+    )
+    assert engine.ask_only("bonjour") == "reponse-brute"
+
+
+def test_concurrent_hotkey_ignored_while_busy(monkeypatch, tmp_path):
+    store = _make_store(tmp_path)
+    engine = AITypewriterEngine(store)
+
+    # Occupe le verrou
+    assert engine._busy.acquire(blocking=False) is True
+    calls = []
+    engine.handle_hotkey()  # doit être ignoré (busy)
+    assert calls == []
+    engine._busy.release()