feat(web): remove devices from the player UI (refs #451)

The merge of soundtouch-web into soundtouch-service was asymmetric:
manual device *adds* propagated to the player UI (HandleAddManualDevice
notifies, the hook re-seeds + broadcasts), but *removals* did not. The
datastore-removal handler never notified, and the web registry's sync
only ever added entries — its map was append-only, so a removed device
lingered in the player UI until restart.

This adds the missing removal path:

- DELETE /api/control/devices/{id} (HandleDeleteDevice). The registry is
  keyed by host/IP; the datastore by device ID (MAC), so the handler
  resolves one to the other via the connection's DeviceInfo, cascades to
  the datastore through a new RemoveDeviceHook (embedded build only),
  prunes the in-memory entry, and broadcasts the updated list.
- WebApp.RemoveDevice prunes the registry and stops the per-device
  goroutines (status poller + WebSocket reconnect loop) via a new
  done-channel + Close() on DeviceConnection — previously both ran for
  the life of the process.
- Server.RemoveDeviceByID extracts the cross-account lookup + remove from
  HandleRemoveDevice and now fires notifyDevicesChanged, so the admin
  Devices tab removal also propagates to the player UI.
- Player UI: a quiet per-card Remove control (visible on hover), a
  confirm dialog, optimistic prune, and a note that a still-online
  device may reappear after the next discovery scan (honest v1 — no
  ignore-list).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/soundtouch-service/main.go

@@ -1125,6 +1125,13 @@ func newEmbeddedWebApp(server *handlers.Server, serverURL string, ds *datastore.
 	// UI "discover" runs the service's sweep, not a second mDNS stack.
 	webApp.TriggerDiscovery = server.DiscoverDevices
 
+	// A removal from the player UI cascades to the datastore (the single
+	// source of truth), so the device does not reappear on the next re-sync.
+	webApp.RemoveDeviceHook = func(deviceID string) error {
+		_, err := server.RemoveDeviceByID(deviceID)
+		return err
+	}
+
 	// Keep the UI registry live as the service discovers or devices are added.
 	server.SetDevicesChangedHook(func() {
 		webApp.SeedExtraDevices()

cmd/soundtouch-service/testdata/router_routes.txt

@@ -4,6 +4,7 @@ DELETE   /accounts/{account}/devices/{device}                         handlers.(
 DELETE   /accounts/{account}/group                                    handlers.(*Server).HandleUnsupported-fm
 DELETE   /accounts/{account}/group/                                   handlers.(*Server).HandleUnsupported-fm
 DELETE   /accounts/{account}/group/{groupId}                          handlers.(*Server).HandleUnsupported-fm
+DELETE   /api/control/devices/{id}/                                   soundtouchweb.(*WebApp).HandleDeleteDevice-fm
 DELETE   /api/setup/devices/{deviceId}                                handlers.(*Server).HandleRemoveDevice-fm
 DELETE   /api/setup/dns-discoveries                                   handlers.(*Server).HandleClearDNSDiscoveries-fm
 DELETE   /api/setup/interactions/sessions                             handlers.(*Server).HandleCleanupSessions-fm