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-core/src/image/mod.rs

@@ -0,0 +1,70 @@
+//! Image drawable.
+
+use crate::{
+    draw_target::DrawTarget,
+    geometry::{OriginDimensions, Point},
+    pixelcolor::PixelColor,
+    primitives::Rectangle,
+};
+
+/// Image drawable.
+///
+/// `ImageDrawable` is implemented for types that contains image information, which makes them
+/// usable with the [`Image`] object.
+///
+/// The methods in this trait shouldn't be called directly by user code. Instead the object
+/// that implements `ImageDrawable` should be wrapped in an [`Image`] object.
+///
+/// # Implementing `ImageDrawable`
+///
+/// All image drawables are positioned at the origin and need to implement [`OriginDimensions`], in
+/// addition to this trait, to define their dimensions.
+///
+/// [`Image`]: https://docs.rs/embedded-graphics/latest/embedded_graphics/image/struct.Image.html
+/// [`OriginDimensions`]: crate::geometry::OriginDimensions
+pub trait ImageDrawable: OriginDimensions {
+    /// The color type.
+    type Color: PixelColor;
+
+    /// Draws the entire image to the target.
+    ///
+    /// This method shouldn't be called directly by user code. Use an [`Image`] object instead.
+    ///
+    /// # Implementation notes
+    ///
+    /// The implementation of this method must draw the image inside the bounding box defined by
+    /// the [`OriginDimensions`] trait implementation. This means that the top left corner is at the
+    /// origin and no drawing operations outside the bounding box are allowed.
+    ///
+    /// [`Image`]: https://docs.rs/embedded-graphics/latest/embedded_graphics/image/struct.Image.html
+    /// [`OriginDimensions`]: crate::geometry::OriginDimensions
+    fn draw<D>(&self, target: &mut D) -> Result<(), D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>;
+
+    /// Draws a part of the image to the target.
+    ///
+    /// This method shouldn't be called directly by user code. Use a [`SubImage`] object instead.
+    ///
+    /// # Implementation notes
+    ///
+    /// The implementation of this method must draw the image inside the given `area`.
+    /// It must be ensured that no drawing operation outside this [`Rectangle`] occur.
+    ///
+    /// [`SubImage`]: https://docs.rs/embedded-graphics/latest/embedded_graphics/image/struct.SubImage.html
+    /// [`Rectangle`]: crate::primitives::rectangle::Rectangle
+    fn draw_sub_image<D>(&self, target: &mut D, area: &Rectangle) -> Result<(), D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>;
+}
+
+/// Pixel getter.
+pub trait GetPixel {
+    /// The color type.
+    type Color: PixelColor;
+
+    /// Gets the color of a pixel.
+    ///
+    /// Returns `None` if `p` is outside the bounding box.
+    fn pixel(&self, p: Point) -> Option<Self::Color>;
+}