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>

Cargo.toml

@@ -20,6 +20,7 @@ serde = { version = "1.0", default-features = false, features = ["derive"] }
 static_cell = "2.1"
 
 # Display rendering
+display-interface = "0.5"
 embedded-graphics = "0.8"
 ssd1306 = { version = "0.10", features = ["async"] }