feat(waybar): add webcam privacy indicator

V4L2 capture goes directly through /dev/videoN and never traverses
the PipeWire portal, so the built-in privacy module misses it. New
custom/webcam module polls fuser on /dev/video* every 2s and shows
a red 'CAM' badge when any device is held open.

Empty text when idle, so the slot collapses and stays out of the way
when the webcam is unused (i.e. always, on a usual day).

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