controller: GET /layouts/{instance} + UI fetch dynamic layout list

UI loadLayouts() теперь fetches /layouts/{inst} — берёт actual layout_map
из config'а (не hardcoded), показывает только existing layouts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

controller/cuda_grid_controller/http_api.py

@@ -85,6 +85,15 @@ def create_app(
         await dispatcher.handle(instance, "layout.set", req.layout)
         return {"ok": True, "instance": instance, "layout": req.layout}
 
+    @app.get("/layouts/{instance}")
+    async def layouts_for_instance(instance: str) -> dict[str, Any]:
+        inst = _check_instance(instance)
+        return {
+            "instance": instance,
+            "layouts": list(inst.layout_map.keys()),
+            "current": await state.get_layout(instance),
+        }
+
     # ─── Overlays ──────────────────────────────────────────────────
 
     @app.post("/overlay/{instance}/add")