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-nb/src/spi.rs

@@ -0,0 +1,44 @@
+//! SPI master mode traits using `nb`.
+
+pub use embedded_hal::spi::{
+    Error, ErrorKind, ErrorType, Mode, Phase, Polarity, MODE_0, MODE_1, MODE_2, MODE_3,
+};
+
+/// Full duplex SPI (master mode).
+///
+/// # Notes
+///
+/// - It's the task of the user of this interface to manage the slave select lines.
+///
+/// - Due to how full duplex SPI works each `read` call must be preceded by a `write` call.
+///
+/// - `read` calls only return the data received with the last `write` call.
+/// Previously received data is discarded
+///
+/// - Data is only guaranteed to be clocked out when the `read` call succeeds.
+/// The slave select line shouldn't be released before that.
+///
+/// - Some SPIs can work with 8-bit *and* 16-bit words. You can overload this trait with different
+/// `Word` types to allow operation in both modes.
+pub trait FullDuplex<Word: Copy = u8>: ErrorType {
+    /// Reads the word stored in the shift register
+    ///
+    /// **NOTE** A word must be sent to the slave before attempting to call this
+    /// method.
+    fn read(&mut self) -> nb::Result<Word, Self::Error>;
+
+    /// Writes a word to the slave
+    fn write(&mut self, word: Word) -> nb::Result<(), Self::Error>;
+}
+
+impl<T: FullDuplex<Word> + ?Sized, Word: Copy> FullDuplex<Word> for &mut T {
+    #[inline]
+    fn read(&mut self) -> nb::Result<Word, Self::Error> {
+        T::read(self)
+    }
+
+    #[inline]
+    fn write(&mut self, word: Word) -> nb::Result<(), Self::Error> {
+        T::write(self, word)
+    }
+}