fix: ack SetConfig/StartRx/StopRx, improve display layout

Protocol:
- SetConfig, StartRx, StopRx now send Response::Ok on success
  (host was timing out waiting for acks that never came)

Display:
- Full frequency precision: 910.525MHz not 910.5MHz
- Full bandwidth labels: 62.5kHz not 62k
- RSSI/SNR row shows units (dBm, dB)
- Separator line moved down 3px to give RSSI/SNR breathing room
- Sparkline is now time-based: each bar = 1 second, scrolls left,
  showing ~1 minute of RSSI history. Empty slots = no packets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/radio/task.rs

@@ -113,6 +113,7 @@ async fn handle_cmd(
         Command::SetConfig(cfg) => {
             state.config = Some(cfg);
             status.sender().send(state.clone());
+            responses.send(Response::Ok).await;
         }
         Command::StartRx => {
             if state.config.is_some() {
@@ -120,6 +121,7 @@ async fn handle_cmd(
                     Ok(()) => {
                         state.state = RadioState::Receiving;
                         status.sender().send(state.clone());
+                        responses.send(Response::Ok).await;
                     }
                     Err(e) => {
                         warn!("StartRx failed: {}", e);
@@ -134,6 +136,7 @@ async fn handle_cmd(
             let _ = lora.enter_standby().await;
             state.state = RadioState::Idle;
             status.sender().send(state.clone());
+            responses.send(Response::Ok).await;
         }
         Command::DisplayOn | Command::DisplayOff => {}
         Command::Transmit { config, payload } => {