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/embedded-io-async/src/impls/vec.rs

@@ -0,0 +1,12 @@
+use alloc::vec::Vec;
+
+use crate::Write;
+
+#[cfg_attr(docsrs, doc(cfg(any(feature = "std", feature = "alloc"))))]
+impl Write for Vec<u8> {
+    #[inline]
+    async fn write(&mut self, buf: &[u8]) -> Result<usize, Self::Error> {
+        self.extend_from_slice(buf);
+        Ok(buf.len())
+    }
+}