feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

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

src/protocol.rs

@@ -55,6 +55,8 @@ pub enum Command {
         config: Option<RadioConfig>,
         payload: Vec<u8, MAX_PAYLOAD>,
     },
+    DisplayOn,
+    DisplayOff,
 }
 
 /// Firmware → host responses.
@@ -70,6 +72,7 @@ pub enum Response {
         payload: Vec<u8, MAX_PAYLOAD>,
     },
     TxDone,
+    Ok,
     Error(ErrorCode),
 }
 
@@ -82,4 +85,5 @@ pub enum ErrorCode {
     TxTimeout = 3,
     CrcError = 4,
     NotConfigured = 5,
+    NoDisplay = 6,
 }