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/usb/task.rs

@@ -1,23 +1,44 @@
 use embassy_executor::task;
 
 use crate::board::UsbParts;
-use crate::channel::{CommandChannel, ResponseChannel};
+use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel};
 
 #[task]
 pub async fn usb_task(
     parts: UsbParts,
     commands: &'static CommandChannel,
     responses: &'static ResponseChannel,
+    display_commands: &'static DisplayCommandChannel,
+    has_display: bool,
 ) {
     let _driver = parts.driver;
     let _ = commands;
     let _ = responses;
+    let _ = display_commands;
+    let _ = has_display;
 
     // TODO: build embassy_usb CDC-ACM class from driver
     // TODO: COBS-framed postcard ser/de loop:
     //   select! {
-    //       bytes = cdc_read() => deserialize Command → commands.send()
-    //       resp  = responses.receive() => serialize Response → cdc_write()
+    //       bytes = cdc_read() => {
+    //           let cmd: Command = deserialize(bytes);
+    //           match cmd {
+    //               Command::DisplayOn => route_display(DisplayCommand::On),
+    //               Command::DisplayOff => route_display(DisplayCommand::Off),
+    //               other => commands.send(other).await,
+    //           }
+    //       }
+    //       resp = responses.receive() => serialize Response → cdc_write()
+    //   }
+    //
+    // Display command routing:
+    //   async fn route_display(cmd, display_commands, has_display, responses) {
+    //       if has_display {
+    //           display_commands.send(cmd).await;
+    //           responses.send(Response::Ok).await;
+    //       } else {
+    //           responses.send(Response::Error(ErrorCode::NoDisplay)).await;
+    //       }
     //   }
 
     loop {