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/channel.rs

@@ -4,6 +4,17 @@ use embassy_sync::watch::Watch;
 
 use crate::protocol::{Command, RadioConfig, Response};
 
+/// Display commands routed from usb_task to display_task.
+#[derive(Debug, Clone, Copy, defmt::Format)]
+#[allow(dead_code)] // Constructed by usb_task once CDC-ACM is wired up
+pub enum DisplayCommand {
+    On,
+    Off,
+}
+
+/// USB-to-display command channel.
+pub type DisplayCommandChannel = Channel<CriticalSectionRawMutex, DisplayCommand, 4>;
+
 /// Host-to-radio command channel.
 pub type CommandChannel = Channel<CriticalSectionRawMutex, Command, 16>;