fix: save button now fully persists canvas to DB

Root cause: handleAddNode/handleEdgeConfirm only updated Zustand store,
never creating records in the DB. canvasApi.save() only updated positions
of existing nodes, so nothing survived a page refresh.

Fix: save now sends the full canvas state (all nodes + edges + data) and
the backend does a full sync — upsert incoming, delete anything removed.
This means Save is the single source of truth: no need to call individual
create/update/delete APIs for every drag or edit.

frontend/src/api/client.ts

@@ -26,8 +26,11 @@ export const authApi = {
 
 export const canvasApi = {
   load: () => api.get('/canvas'),
-  save: (payload: { node_positions: { id: string; x: number; y: number }[]; viewport: object }) =>
-    api.post('/canvas/save', payload),
+  save: (payload: {
+    nodes: object[]
+    edges: object[]
+    viewport: object
+  }) => api.post('/canvas/save', payload),
 }
 
 export const nodesApi = {