feat(bookslm): v2.2.0 - BooksLM chat AI contextuel par répertoire + 4 providers AI

BooksLM (#76):
- Panneau chat slide-in 450px, clic-droit répertoire → 🧠 BooksLM
- Collecte récursive .md avec limites (200 fichiers, 200K chars)
- Cache SHA-256, redaction secrets, priorité README/index
- SSE streaming, badges sources cliquables, historique localStorage
- Commandes palette: BooksLM ouvrir/nouvelle conversation

Providers AI (4 nouveaux):
- NVIDIA (integrate.api.nvidia.com)
- QwenCloud (dashscope.aliyuncs.com)
- Xiaomi (api.xiaomi.com)
- Mistral (api.mistral.ai)
- Tous OpenAI-compatible, auto-listing modèles

Fix: dropdown config-select suit maintenant le thème (option bg/color)
27 tests BooksLM + 466 tests au total

backend/main.py

@@ -695,6 +695,7 @@ except Exception:  # pragma: no cover - WeasyPrint/GTK missing
 # Multi-format export (HTML / MD bundle / ePub) — pure Python, no heavy deps.
 from backend.export import export_epub, export_html, export_md_bundle, ExportError  # noqa: E402
 from backend.ai_routes import router as ai_router
+from backend.bookslm_routes import router as bookslm_router
 from backend.saved_searches import delete_saved, get_saved, save_search
 from backend.share import (
     create_share,
@@ -714,6 +715,7 @@ from backend.webhooks import (
 
 app.include_router(auth_router)
 app.include_router(ai_router)
+app.include_router(bookslm_router)
 
 # Resolve frontend path relative to this file
 FRONTEND_DIR = Path(__file__).resolve().parent.parent / "frontend"
@@ -3993,7 +3995,7 @@ async def api_get_ai_keys(current_user=Depends(require_admin)):
     """Return stored AI keys (values masked)."""
     keys = _read_ai_keys()
     masked = {}
-    for k in ["DEEPSEEK_API_KEY", "OPENROUTER_API_KEY", "GEMINI_API_KEY"]:
+    for k in ["DEEPSEEK_API_KEY", "OPENROUTER_API_KEY", "GEMINI_API_KEY", "NVIDIA_API_KEY", "QWENCLOUD_API_KEY", "XIAOMI_API_KEY", "MISTRAL_API_KEY"]:
         val = keys.get(k, "") or os.environ.get(k, "")
         if val:
             masked[k] = val[:4] + "..." + val[-4:] if len(val) > 8 else "***"
@@ -4005,7 +4007,7 @@ async def api_get_ai_keys(current_user=Depends(require_admin)):
 async def api_set_ai_keys(body: dict = Body(...), current_user=Depends(require_admin)):
     """Save AI keys. Pass {"DEEPSEEK_API_KEY":"sk-...","OPENROUTER_API_KEY":"...","GEMINI_API_KEY":"..."}"""
     keys = _read_ai_keys()
-    for k in ["DEEPSEEK_API_KEY", "OPENROUTER_API_KEY", "GEMINI_API_KEY"]:
+    for k in ["DEEPSEEK_API_KEY", "OPENROUTER_API_KEY", "GEMINI_API_KEY", "NVIDIA_API_KEY", "QWENCLOUD_API_KEY", "XIAOMI_API_KEY", "MISTRAL_API_KEY"]:
         if body.get(k):
             keys[k] = body[k]
     _write_ai_keys(keys)
@@ -4020,6 +4022,10 @@ async def api_test_ai_keys(current_user=Depends(require_admin)):
         ("DEEPSEEK_API_KEY", "deepseek", "https://api.deepseek.com/v1/models", "Authorization"),
         ("OPENROUTER_API_KEY", "openrouter", "https://openrouter.ai/api/v1/models", "Authorization"),
         ("GEMINI_API_KEY", "gemini", "https://generativelanguage.googleapis.com/v1beta/models?key={key}", None),
+        ("NVIDIA_API_KEY", "nvidia", "https://integrate.api.nvidia.com/v1/models", "Authorization"),
+        ("QWENCLOUD_API_KEY", "qwencloud", "https://dashscope.aliyuncs.com/compatible-mode/v1/models", "Authorization"),
+        ("XIAOMI_API_KEY", "xiaomi", "https://api.xiaomi.com/v1/models", "Authorization"),
+        ("MISTRAL_API_KEY", "mistral", "https://api.mistral.ai/v1/models", "Authorization"),
     ]:
         key = get_ai_key(key_name)
         if not key:
@@ -4047,7 +4053,8 @@ async def api_list_ai_models(provider: str = Query(...), current_user=Depends(re
     """List available models for a given AI provider."""
     provider = provider.lower()
 
-    if provider not in ("deepseek", "openrouter", "gemini"):
+    all_providers = ("deepseek", "openrouter", "gemini", "nvidia", "qwencloud", "xiaomi", "mistral")
+    if provider not in all_providers:
         return {"models": [], "error": f"Unknown provider: {provider}"}
 
     key_name = f"{provider.upper()}_API_KEY"
@@ -4060,8 +4067,16 @@ async def api_list_ai_models(provider: str = Query(...), current_user=Depends(re
         url = f"https://generativelanguage.googleapis.com/v1beta/models?key={key}"
     elif provider == "deepseek":
         url = "https://api.deepseek.com/v1/models"
-    else:  # openrouter
+    elif provider == "openrouter":
         url = "https://openrouter.ai/api/v1/models"
+    elif provider == "nvidia":
+        url = "https://integrate.api.nvidia.com/v1/models"
+    elif provider == "qwencloud":
+        url = "https://dashscope.aliyuncs.com/compatible-mode/v1/models"
+    elif provider == "xiaomi":
+        url = "https://api.xiaomi.com/v1/models"
+    elif provider == "mistral":
+        url = "https://api.mistral.ai/v1/models"
 
     try:
         if provider == "gemini":