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/once_cell/examples/reentrant_init_deadlocks.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let cell = once_cell::sync::OnceCell::<u32>::new();
+    cell.get_or_init(|| {
+        cell.get_or_init(|| 1);
+        2
+    });
+}
+
+/// Dummy test to make it seem hang when compiled as `--test`
+/// See https://github.com/matklad/once_cell/issues/79
+#[test]
+fn dummy_test() {
+    std::thread::sleep(std::time::Duration::from_secs(4));
+}