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/ssd1306/examples/pixelsquare.rs

@@ -0,0 +1,86 @@
+//! This example draws a small square one pixel at a time in the top left corner of the display
+//!
+//! You will probably want to use the [`embedded_graphics`](https://crates.io/crates/embedded-graphics) crate to do more complex drawing.
+//!
+//! This example is for the STM32F103 "Blue Pill" board using a 4 wire interface to the display on
+//! SPI1.
+//!
+//! Wiring connections are as follows
+//!
+//! ```
+//! GND -> GND
+//! 3V3 -> VCC
+//! PA5 -> SCL (D0)
+//! PA7 -> SDA (D1)
+//! PB0 -> RST
+//! PB1 -> D/C
+//! PB10 -> CS
+//! ```
+//!
+//! Run on a Blue Pill with `cargo run --example pixelsquare`.
+
+#![no_std]
+#![no_main]
+
+use cortex_m::asm::nop;
+use cortex_m_rt::entry;
+use defmt_rtt as _;
+use embassy_stm32::{
+    gpio,
+    spi::{self, Spi},
+    time::Hertz,
+};
+use panic_probe as _;
+use ssd1306::{prelude::*, Ssd1306};
+
+#[entry]
+fn main() -> ! {
+    let p = embassy_stm32::init(Default::default());
+    let mut config = spi::Config::default();
+    config.frequency = Hertz::mhz(8);
+    let spi = Spi::new_blocking_txonly(p.SPI1, p.PA5, p.PA7, config);
+
+    let mut rst = gpio::Output::new(p.PB0, gpio::Level::Low, gpio::Speed::Low);
+    let dc = gpio::Output::new(p.PB1, gpio::Level::Low, gpio::Speed::Low);
+    let cs = gpio::Output::new(p.PB10, gpio::Level::Low, gpio::Speed::Low);
+    let spi = embedded_hal_bus::spi::ExclusiveDevice::new_no_delay(spi, cs).unwrap();
+
+    let interface = display_interface_spi::SPIInterface::new(spi, dc);
+    let mut display = Ssd1306::new(interface, DisplaySize128x64, DisplayRotation::Rotate0)
+        .into_buffered_graphics_mode();
+
+    display
+        .reset(&mut rst, &mut embassy_time::Delay {})
+        .unwrap();
+    display.init().unwrap();
+
+    // Top side
+    display.set_pixel(0, 0, true);
+    display.set_pixel(1, 0, true);
+    display.set_pixel(2, 0, true);
+    display.set_pixel(3, 0, true);
+
+    // Right side
+    display.set_pixel(3, 0, true);
+    display.set_pixel(3, 1, true);
+    display.set_pixel(3, 2, true);
+    display.set_pixel(3, 3, true);
+
+    // Bottom side
+    display.set_pixel(0, 3, true);
+    display.set_pixel(1, 3, true);
+    display.set_pixel(2, 3, true);
+    display.set_pixel(3, 3, true);
+
+    // Left side
+    display.set_pixel(0, 0, true);
+    display.set_pixel(0, 1, true);
+    display.set_pixel(0, 2, true);
+    display.set_pixel(0, 3, true);
+
+    display.flush().unwrap();
+
+    loop {
+        nop()
+    }
+}