feat: prompt on duplicate device instead of silently blocking/merging approve

Single-device approve now guards duplicates per-design the same way bulk
approve does, and asks the user instead of failing or silently merging:

- create_node and approve_device reject a same-design duplicate (ieee, ip
  or mac) with 409 + the existing node; a force flag creates it anyway.
- Frontend shows a confirm dialog: go to existing node, add duplicate
  anyway, or cancel.
- approve_device no longer rejects a device already on another canvas
  (status is global, canvas membership is per-design) — it can be placed
  on a new design, matching bulk approve.
- IEEE (Zigbee/Z-Wave) devices now use the same prompt as ip/mac instead
  of auto-merging into the existing node.
- bulk approve reports which devices it skipped as duplicates.

Closes #260

ha-relevant: maybe

backend/app/schemas/nodes.py

@@ -39,6 +39,10 @@ class NodeBase(BaseModel):
 
 class NodeCreate(NodeBase):
     design_id: str | None = None
+    # When a node with the same ip/mac already exists on the target design, the
+    # create/approve endpoints reject with 409 so the UI can ask the user. Set
+    # force=True to bypass that guard and create the duplicate deliberately.
+    force: bool = False
 
 
 class NodeUpdate(BaseModel):