fix(liveview): nest docker children, apply theme & custom style

- Live view rebuilt parent map from proxmox/group only, so docker_container
  (and any vm/lxc with container_mode=true) lost parentId and rendered
  outside its host. Match App.tsx logic: include any node with
  container_mode=true.
- Live view ignored viewport.theme_id and custom_style on load, so the
  shared canvas always rendered in default theme. Apply both like the
  authenticated canvas does.
- Backend /liveview response now includes custom_style from CanvasState.

Adds regression tests on both ends.

backend/app/api/routes/liveview.py

@@ -34,8 +34,10 @@ async def liveview_canvas(
     edges = (await db.execute(select(Edge))).scalars().all()
     state = await db.get(CanvasState, 1)
     viewport: dict[str, Any] = state.viewport if state else {"x": 0, "y": 0, "zoom": 1}
+    custom_style: dict[str, Any] | None = state.custom_style if state else None
     return CanvasStateResponse(
         nodes=[NodeResponse.model_validate(n) for n in nodes],
         edges=[EdgeResponse.model_validate(e) for e in edges],
         viewport=viewport,
+        custom_style=custom_style,
     )