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