NX Sentry 0.4.0

Three additions, all aimed at watching a room through a webcam.

Ignore areas. Draw rectangles over the parts inside your selection that
always move but never matter — a clock, a timestamp burnt into the image, a tree
in the corner. Their pixels stop counting: they are subtracted from the
measurement, not just hidden, so the percentage you read is a share of what is
actually being watched, and the outline of "what moved" never grows around them.
They stay pinned to the frame, so you can move and resize the watched rectangle
around them freely.

Cameras. Watch a webcam, a capture card or a virtual device directly — V4L2
on Linux, through the same capture stack. No portal dialog, which is why the
last camera can be reopened automatically on launch (a screen share cannot be
resumed that way without asking permission at every start). Cameras are matched
by device id, never by position, so unplugging one cannot promote another into
its place.

A picture of what tripped it. Every trigger keeps a snapshot of the watched
area from the frame that fired, with the moving part outlined and ignored parts
marked. Click any event to open the viewer: arrow keys step through, Play runs
them as a slideshow, and each carries its timestamp and pixel count. They live
in memory — the last 24 — and are written to disk only if you turn that on, in
which case they go to ~/.config/nx-sentry/snapshots and nowhere else.

Verify the download against the .sha256 sidecar; the .sig is ed25519 over
that digest with the NX release key NX Hub pins.