feat(ai): menu @ instantane, selecteurs agrandis et BooksLM racine (#82)

Nouvel endpoint GET /api/vault/{vault}/paths + prechargement et filtrage client du menu @ (affichage instantane des fichiers/repertoires). Selecteurs Fournisseur/Modele agrandis (0,8rem / 34px) pour s'aligner sur le reste du site. Ajout de BooksLM au menu contextuel de la racine des vaults.

tests/test_api_main.py

@@ -312,6 +312,22 @@ class TestTreeSearch:
         data = resp.json()
         assert len(data["results"]) == 0
 
+    def test_vault_paths_returns_indexed_paths(self, client):
+        """Flat path index backs the AI assistant `@` mention menu."""
+        resp = client.get("/api/vault/TestVault/paths")
+        assert resp.status_code == 200
+        data = resp.json()
+        assert data["vault"] == "TestVault"
+        paths = [p["path"] for p in data["results"]]
+        assert "note1.md" in paths
+        assert all("type" in p for p in data["results"])
+
+    def test_vault_paths_rejects_unknown_vault(self, client):
+        resp = client.get("/api/vault/DoesNotExist/paths")
+        assert resp.status_code in (200, 403, 404)
+        if resp.status_code == 200:
+            assert resp.json()["results"] == []
+
 
 class TestSearchReplace:
     def test_search_replace_invalid(self, client):