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/main.rs

@@ -10,7 +10,7 @@ mod usb;
 
 use embassy_executor::Spawner;
 
-use crate::channel::{CommandChannel, ResponseChannel, StatusWatch};
+use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel, StatusWatch};
 
 cfg_if::cfg_if! {
     if #[cfg(feature = "rak_4631")] {
@@ -23,16 +23,19 @@ cfg_if::cfg_if! {
 static COMMANDS: CommandChannel = CommandChannel::new();
 static RESPONSES: ResponseChannel = ResponseChannel::new();
 static STATUS: StatusWatch = StatusWatch::new();
+static DISPLAY_COMMANDS: DisplayCommandChannel = DisplayCommandChannel::new();
 
 #[embassy_executor::main]
 async fn main(spawner: Spawner) {
     let board = board::Board::init();
     let (radio, usb, display) = board.into_parts();
 
+    let has_display = display.is_some();
+
     spawner.spawn(radio::radio_task(radio, &COMMANDS, &RESPONSES, &STATUS)).unwrap();
-    spawner.spawn(usb::usb_task(usb, &COMMANDS, &RESPONSES)).unwrap();
+    spawner.spawn(usb::usb_task(usb, &COMMANDS, &RESPONSES, &DISPLAY_COMMANDS, has_display)).unwrap();
 
     if let Some(dp) = display {
-        spawner.spawn(display::display_task(dp, &STATUS)).unwrap();
+        spawner.spawn(display::display_task(dp, &STATUS, &DISPLAY_COMMANDS)).unwrap();
     }
 }