[client/ui] Add async Up mode to avoid blocking profile switches

The daemon's Up RPC previously always blocked in waitForUp (up to 50s)
until the engine connected. The UI does not need this — status updates
already flow through the SubscribeStatus stream.

Add bool async = 4 to UpRequest. When true the daemon starts
connectWithRetryRuns and returns immediately; the CLI path (async=false,
the default) is unchanged.

ProfileSwitcher.SwitchActive now sets Async:true so all three RPCs
(Status, Switch, Down, Up) return quickly. The background goroutine and
its associated race condition are removed entirely.

client/proto/daemon.proto

@@ -233,6 +233,12 @@ message UpRequest {
   optional string profileName = 1;
   optional string username = 2;
   reserved 3;
+  // async instructs the daemon to start the connection attempt and return
+  // immediately without waiting for the engine to become ready. Status updates
+  // are delivered via the SubscribeStatus stream. When false (the default) the
+  // RPC blocks until the engine is running or gives up, which is the behaviour
+  // needed by the CLI.
+  bool async = 4;
 }
 
 message UpResponse {}