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-hal/src/lib.rs

@@ -0,0 +1,21 @@
+#![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
+#![no_std]
+
+pub mod delay;
+pub mod digital;
+pub mod i2c;
+pub mod pwm;
+pub mod spi;
+
+mod private {
+    use crate::i2c::{SevenBitAddress, TenBitAddress};
+    pub trait Sealed {}
+
+    impl Sealed for SevenBitAddress {}
+    impl Sealed for TenBitAddress {}
+}
+
+// needed to prevent defmt macros from breaking, since they emit code that does `defmt::blahblah`.
+#[cfg(feature = "defmt-03")]
+use defmt_03 as defmt;