feat: Device Inventory — show all scanned devices with canvas-presence

Reworks the "Pending Devices" panel into a "Device Inventory": scanned
devices already placed on a canvas are no longer suppressed — they stay
listed and badged with how many canvases they appear on.

- scanner: stop deleting/skipping on-canvas IPs (hidden still suppressed)
- scan API: /pending returns all non-hidden devices; compute canvas_count
  by correlating ip/ieee_address against nodes grouped by design
- frontend: rename to "Device Inventory", top-right canvas-count corner,
  toggle to show/hide on-canvas devices (default show)

ha-relevant: maybe

backend/app/schemas/scan.py

@@ -21,6 +21,9 @@ class PendingDeviceResponse(BaseModel):
     vendor: str | None = None
     lqi: int | None = None
     discovered_at: datetime
+    # Number of distinct canvases (designs) this device already appears on,
+    # correlated by ip / ieee_address against existing nodes. Computed per-request.
+    canvas_count: int = 0
 
     model_config = {"from_attributes": True}