Add peer-to-peer file drop

Files move directly between peers over the overlay, with no server in the
path. The receiver listens on the WireGuard address only, so the port is
unreachable from outside the tunnel, and every offer is matched to a known
peer before anything is read.

Consent is the default: an offer carries metadata alone, and no payload
moves until the receiver accepts. Policy is per profile and device-local —
off, ask, or auto-accept, with per-sender exceptions on top.

Policy and history live in the profile's preferences, so removing a profile
takes its file drop state with it. Transfers interrupted by a restart are
settled on load; nothing survives to finish them, and left alone they would
sit in the log as permanently pending.

The Android bindings pull payload bytes through a chunk-returning stream:
gomobile copies a []byte argument into a fresh Java array and never copies
it back, so a fill-my-buffer method would hand back the right length with
no data.

client/ui/tray.go

@@ -41,6 +41,7 @@ const (
 // stays under the linter's parameter-count threshold.
 type TrayServices struct {
 	Connection      *services.Connection
+	FileDrop        *services.FileDrop
 	Settings        *services.Settings
 	Profiles        *services.Profiles
 	Networks        *services.Networks
@@ -200,6 +201,7 @@ func NewTray(app *application.App, window *application.WebviewWindow, svc TraySe
 
 	app.Event.On(services.EventStatusSnapshot, t.onStatusEvent)
 	app.Event.On(services.EventDaemonNotification, t.onSystemEvent)
+	app.Event.On(services.EventFileDrop, t.onFileDropEvent)
 	// Refresh the Profiles submenu on ProfileSwitcher's change event. A
 	// switch on an idle daemon drives no status transition, so without this
 	// hook a React-initiated switch leaves the tray's submenu stale.
@@ -217,6 +219,8 @@ func NewTray(app *application.App, window *application.WebviewWindow, svc TraySe
 		// Startup populates appName/registry path on Windows; before app.Run()
 		// the category lookup silently falls back to a plain notification.
 		t.registerSessionWarningCategory()
+		t.registerFileDropCategory()
+		t.registerNotificationResponses()
 	})
 
 	t.loc.Watch(func(i18n.LanguageCode) { t.applyLanguage() })