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-graphics/benches/image.rs

@@ -0,0 +1,25 @@
+use criterion::*;
+use embedded_graphics::{
+    image::{Image, ImageRaw},
+    pixelcolor::BinaryColor,
+    prelude::*,
+};
+
+mod common;
+
+use common::Framebuffer;
+
+fn image_1bpp(c: &mut Criterion) {
+    c.bench_function("image 4x4px", |b| {
+        let bytes = include_bytes!("../assets/patch_1bpp.raw");
+
+        let image: ImageRaw<BinaryColor> = ImageRaw::new(bytes, 4);
+        let object = Image::new(&image, Point::zero());
+
+        let mut framebuffer = Framebuffer::new();
+        b.iter(|| object.draw(&mut framebuffer))
+    });
+}
+
+criterion_group!(images, image_1bpp);
+criterion_main!(images);