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/embassy-embedded-hal/src/lib.rs

@@ -0,0 +1,39 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+#![allow(async_fn_in_trait)]
+#![warn(missing_docs)]
+#![doc = include_str!("../README.md")]
+
+pub mod adapter;
+pub mod flash;
+pub mod shared_bus;
+
+/// Set the configuration of a peripheral driver.
+///
+/// This trait is intended to be implemented by peripheral drivers such as SPI
+/// and I2C. It allows changing the configuration at runtime.
+///
+/// The exact type of the "configuration" is defined by each individual driver, since different
+/// drivers support different options. Therefore it is defined as an associated type.
+///
+/// For example, it is used by [`SpiDeviceWithConfig`](crate::shared_bus::asynch::spi::SpiDeviceWithConfig) and
+///  [`I2cDeviceWithConfig`](crate::shared_bus::asynch::i2c::I2cDeviceWithConfig) to allow different
+/// devices on the same bus to use different communication settings.
+pub trait SetConfig {
+    /// The configuration type used by this driver.
+    type Config;
+
+    /// The error type that can occur if `set_config` fails.
+    type ConfigError;
+
+    /// Set the configuration of the driver.
+    fn set_config(&mut self, config: &Self::Config) -> Result<(), Self::ConfigError>;
+}
+
+/// Get the configuration of a peripheral driver.
+pub trait GetConfig {
+    /// The configuration type used by this driver.
+    type Config;
+
+    /// Get the configuration of the driver.
+    fn get_config(&self) -> Self::Config;
+}