feat: resizable nodes with width/height persistence

Add NodeResizer to BaseNode so users can drag corners to resize any node.
Persist width/height through the full stack: DB model, schemas, canvas
save/load route, and migration for existing databases.

Add tests covering save, update, clear, and load of node dimensions.

backend/app/schemas/canvas.py

@@ -28,6 +28,8 @@ class NodeSave(BaseModel):
     ram_gb: float | None = None
     disk_gb: float | None = None
     show_hardware: bool = False
+    width: float | None = None
+    height: float | None = None
     pos_x: float = 0
     pos_y: float = 0
 

backend/app/schemas/nodes.py

@@ -27,6 +27,8 @@ class NodeBase(BaseModel):
     ram_gb: float | None = None
     disk_gb: float | None = None
     show_hardware: bool = False
+    width: float | None = None
+    height: float | None = None
 
 
 class NodeCreate(NodeBase):
@@ -56,6 +58,8 @@ class NodeUpdate(BaseModel):
     ram_gb: float | None = None
     disk_gb: float | None = None
     show_hardware: bool | None = None
+    width: float | None = None
+    height: float | None = None
 
 
 class NodeResponse(NodeBase):