Find the camera on the network instead of asking for its address

The add-camera form asked for an IP address, and a shop owner does not
know their camera's IP address - it is on a sticker under the camera or
in a menu that differs by make. That field is where onboarding stopped
for anyone who was not an installer.

behavision/discover.py: one ONVIF WS-Discovery multicast (names the
camera and often its make) merged with a TCP sweep of port 554 across
the local /24 (misses nothing that streams). Stdlib only, ~4 s on the
office network, both cameras found. The add-camera sheet leads with
'Find cameras on this network'; picking a row fills the address and,
when the make is recognisable, the stream path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj

desktop/app.go

@@ -609,6 +609,14 @@ func (a *App) Cameras() ([]map[string]any, error) {
 	return a.local.Cameras(ctx)
 }
 
+// DiscoverCameras lists the cameras on this PC's network, so the add-camera
+// form is a pick-list and not a request for an IP address nobody knows.
+func (a *App) DiscoverCameras() (map[string]any, error) {
+	ctx, cancel := context.WithTimeout(a.ctx, 30*time.Second)
+	defer cancel()
+	return a.local.DiscoverCameras(ctx)
+}
+
 func (a *App) TestCamera(cam map[string]any) (map[string]any, error) {
 	ctx, cancel := context.WithTimeout(a.ctx, 60*time.Second)
 	defer cancel()