[client] Push-based status stream for the Wails UI

Adds a SubscribeStatus gRPC RPC that pushes a fresh FullStatus snapshot
on every peer-recorder state change, replacing the Wails UI's 2-second
Status poll. The daemon's notifier already triggers on Connected /
Disconnected / Connecting / management or signal flip / address
change / peers-list change; we now coalesce those into ticks on a
buffered chan and stream the resulting snapshots over gRPC.

- Status recorder gains SubscribeToStateChanges /
  UnsubscribeFromStateChanges + a non-blocking notifyStateChange that
  drops ticks when a subscriber's 1-slot buffer is full (next snapshot
  the consumer pulls already reflects everything).
- Server.Status handler split: the snapshot composition is shared
  with the new SubscribeStatus stream handler so unary and stream
  paths return identical bytes.
- UI peers service: pollLoop replaced by statusStreamLoop. The local
  name of the existing SubscribeEvents loop is now toastStreamLoop so
  the two streams are easy to tell apart — the underlying RPC name is
  unchanged.
- Tray applyStatus skips the icon refresh when connected/lastStatus
  hasn't changed; rapid SubscribeStatus bursts during health probes
  no longer churn Shell_NotifyIcon or the log.

client/proto/daemon.proto

@@ -24,6 +24,12 @@ service DaemonService {
   // Status of the service.
   rpc Status(StatusRequest) returns (StatusResponse) {}
 
+  // SubscribeStatus pushes a fresh StatusResponse on connection state
+  // changes (Connected / Disconnected / Connecting / address change /
+  // peers list change). The first message on the stream is the current
+  // snapshot, so a freshly-subscribed UI doesn't need to also call Status.
+  rpc SubscribeStatus(StatusRequest) returns (stream StatusResponse) {}
+
   // Down stops engine work in the daemon.
   rpc Down(DownRequest) returns (DownResponse) {}