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/tests/custom_primitive.rs

@@ -0,0 +1,75 @@
+use embedded_graphics::{
+    mock_display::MockDisplay,
+    pixelcolor::Rgb888,
+    prelude::*,
+    primitives::{PrimitiveStyle, Rectangle, StyledDimensions, StyledDrawable},
+};
+
+struct Square {
+    top_left: Point,
+    size: u32,
+}
+
+impl Square {
+    fn new(top_left: Point, size: u32) -> Self {
+        Self { top_left, size }
+    }
+
+    fn to_rectangle(&self) -> Rectangle {
+        Rectangle::new(self.top_left, Size::new_equal(self.size))
+    }
+}
+
+impl Primitive for Square {}
+
+impl Dimensions for Square {
+    fn bounding_box(&self) -> Rectangle {
+        self.to_rectangle()
+    }
+}
+
+impl<C: PixelColor> StyledDrawable<PrimitiveStyle<C>> for Square {
+    type Color = C;
+    type Output = ();
+
+    fn draw_styled<D>(
+        &self,
+        style: &PrimitiveStyle<C>,
+        target: &mut D,
+    ) -> Result<Self::Output, D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>,
+    {
+        self.to_rectangle().draw_styled(style, target)
+    }
+}
+
+impl<C: PixelColor> StyledDimensions<PrimitiveStyle<C>> for Square {
+    fn styled_bounding_box(&self, style: &PrimitiveStyle<C>) -> Rectangle {
+        self.to_rectangle().styled_bounding_box(style)
+    }
+}
+
+#[test]
+fn draw_custom_primitive() {
+    let mut display = MockDisplay::new();
+    Square::new(Point::new(1, 0), 2)
+        .into_styled(PrimitiveStyle::with_fill(Rgb888::RED))
+        .draw(&mut display)
+        .unwrap();
+    display.assert_pattern(&[
+        " RR", //
+        " RR", //
+    ]);
+}
+
+#[test]
+fn custom_primitive_dimensions() {
+    let styled_square =
+        Square::new(Point::new(1, 0), 2).into_styled(PrimitiveStyle::with_stroke(Rgb888::RED, 2));
+
+    assert_eq!(
+        styled_square.bounding_box(),
+        Rectangle::new(Point::new(1, 0), Size::new_equal(2)).offset(1)
+    );
+}