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>

vendor/thiserror/tests/ui/concat-display.rs

@@ -0,0 +1,15 @@
+use thiserror::Error;
+
+macro_rules! error_type {
+    ($name:ident, $what:expr) => {
+        // Use #[error("invalid {}", $what)] instead.
+
+        #[derive(Error, Debug)]
+        #[error(concat!("invalid ", $what))]
+        pub struct $name;
+    };
+}
+
+error_type!(Error, "foo");
+
+fn main() {}