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/radio/task.rs

@@ -66,6 +66,9 @@ pub async fn radio_task(
                 // TODO: put lora radio into standby
                 status.sender().send(state.clone());
             }
+            Command::DisplayOn | Command::DisplayOff => {
+                // Routed by usb_task to display — should never reach radio.
+            }
             Command::Transmit { config, payload } => {
                 let tx_config = config.or(state.config);
                 if let Some(_cfg) = tx_config {