feat(api,ai): #72 OpenAPI 3.1 enrichie + fiabilisation de l'outil AI UI

API (#72):
- backend/openapi_docs.py: 18 tags documentes, assignation auto par prefixe,
  securite bearerAuth/cookieAuth, erreurs 401/403/404/422/500, exemples,
  page /api autonome (liens /docs, /redoc, /openapi.json)
- backend/schemas.py: response_model Pydantic pour ~35 endpoints sans modele
- main.py: FastAPI enrichi + override app.openapi; routes /api et /api/
- ai_routes/bookslm_routes: response_model + doc SSE
- frontend: entree 'API' du menu (i18n FR/EN)
- tests/test_openapi.py (58 tests)

AI UI:
- BooksLM: requetes authentifiees (AuthManager), parsing SSE {token}/error,
  message utilisateur correct (plus le placeholder vide), barre de contexte
- AI Editor: Ctrl/Cmd+J lie une seule fois, libelles i18n, modale de
  reecriture accessible a la place de window.prompt()
- tests/frontend/ai.test.mjs (7 tests) + CI

frontend/style.css

@@ -2675,6 +2675,88 @@ select {
   50% { opacity: 0.6; }
 }
 
+/* AI custom-rewrite modal — replaces the browser prompt() for a consistent,
+   theme-aware and keyboard-accessible experience. */
+.ai-modal-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 10050;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 20px;
+  background: rgba(0, 0, 0, 0.55);
+  backdrop-filter: blur(2px);
+}
+
+.ai-modal {
+  width: 100%;
+  max-width: 460px;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border);
+  border-radius: 10px;
+  padding: 18px 18px 14px;
+  box-shadow: 0 16px 48px rgba(0, 0, 0, 0.45);
+}
+
+.ai-modal-title {
+  margin: 0 0 4px;
+  font-size: 0.95rem;
+  color: var(--text-primary);
+}
+
+.ai-modal-hint {
+  margin: 0 0 10px;
+  font-size: 0.78rem;
+  color: var(--text-muted);
+}
+
+.ai-modal-input {
+  width: 100%;
+  resize: vertical;
+  min-height: 72px;
+  padding: 8px 10px;
+  font-family: inherit;
+  font-size: 0.85rem;
+  color: var(--text-primary);
+  background: var(--bg-primary);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+}
+
+.ai-modal-input:focus {
+  outline: none;
+  border-color: var(--accent);
+  box-shadow: 0 0 0 2px rgba(96, 165, 250, 0.25);
+}
+
+.ai-modal-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+  margin-top: 12px;
+}
+
+.ai-modal-btn {
+  padding: 6px 14px;
+  font-size: 0.8rem;
+  border-radius: 6px;
+  cursor: pointer;
+  border: 1px solid var(--border);
+  background: transparent;
+  color: var(--text-primary);
+}
+
+.ai-modal-btn:hover { background: var(--bg-hover); }
+
+.ai-modal-btn.primary {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: #fff;
+}
+
+.ai-modal-btn.primary:hover { filter: brightness(1.08); }
+
 .editor-title {
   font-family: "JetBrains Mono", monospace;
   font-size: 0.9rem;