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/README.md

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+<p align="center">
+    <img width="150" src="https://raw.githubusercontent.com/embedded-graphics/embedded-graphics/b225511f390c0ed9bc065eb67d05125845312148/assets/logo_core.svg?sanitize=true" alt="Embedded graphics logo">
+</p>
+<p align="center">
+    <a href="https://circleci.com/gh/embedded-graphics/embedded-graphics/tree/master"><img src="https://circleci.com/gh/embedded-graphics/embedded-graphics/tree/master.svg?style=shield" alt="Build Status"></a>
+    <a href="https://crates.io/crates/embedded-graphics-core"><img src="https://img.shields.io/crates/v/embedded-graphics-core.svg" alt="Crates.io"></a>
+    <a href="https://docs.rs/embedded-graphics-core"><img src="https://docs.rs/embedded-graphics-core/badge.svg" alt="Docs.rs"></a>
+    <a href="https://matrix.to/#/#rust-embedded-graphics:matrix.org"><img src="https://img.shields.io/matrix/rust-embedded-graphics:matrix.org" alt="embedded-graphics on Matrix"></a>
+</p>
+
+# Embedded graphics core
+
+embedded-graphics-core contains the core components of [embedded-graphics] that are required to
+add embedded-graphics support to display drivers, image libraries, text renderers and other
+third party crates.
+
+This crate should only be used by crates that extend embedded-graphics.
+Applications should instead depend on [embedded-graphics] itself.
+
+Like any other crate, embedded-graphics-core will change over time, however it will change at a
+much slower rate than embedded-graphics itself, and will likely release fewer breaking changes.
+This will provide more stability and compatibility for the wider embedded-graphics ecosystem,
+whilst allowing non-core features of embedded-graphics to evolve at a faster pace. The same
+version of embedded-graphics-core may be used for multiple major versions of embedded-graphics.
+
+### Core functionality
+
+* [`DrawTarget`] - By implementing a draw target for a display driver, all embedded-graphics drawables can be drawn to that display.
+* [`Drawable`] - This trait can be implemented to make an object drawable to any [`DrawTarget`]. Examples include shapes, text, UI elements, etc.
+* [`ImageDrawable`]
+* Color types - see below.
+* Geometry - [`Point`], [`Size`] and [`Rectangle`] provide ways of defining positions, dimensions and rectangular areas respectively.
+
+## Colors
+
+The [`pixelcolor`] module provides various standard color types, from [`BinaryColor`] to
+[`Rgb888`]. See the [`pixelcolor`] module documentation for the complete list of color depths
+and formats available.
+
+## Display drivers
+
+See the [`DrawTarget`] documentation for examples on how to integrate embedded-graphics with a
+display driver using the [`DrawTarget`] trait.
+
+## Images
+
+The [`ImageDrawable`] trait should be implemented for any image or image-like item, for example
+a spritemap.
+
+[`Pixel`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/drawable/struct.Pixel.html
+[`Point`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/geometry/struct.Point.html
+[`Size`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/geometry/struct.Size.html
+[`Drawable`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/drawable/trait.Drawable.html
+[`DrawTarget`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/draw_target/trait.DrawTarget.html
+[`Rectangle`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/primitives/rectangle/struct.Rectangle.html
+[`Dimensions`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/geometry/trait.Dimensions.html
+[`OriginDimensions`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/geometry/trait.OriginDimensions.html
+[`BinaryColor`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/pixelcolor/enum.BinaryColor.html
+[`Rgb888`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/pixelcolor/struct.Rgb888.html
+[`ImageDrawable`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/image/trait.ImageDrawable.html
+[`pixelcolor`]: https://docs.rs/embedded-graphics-core/latest/embedded_graphics_core/pixelcolor/trait.PixelColor.html
+
+## Features
+
+ - `defmt` - implements `defmt::Format` for all types where this is possible. Note that `defmt`
+   might not work with older versions of rustc that are otherwise supported by embedded-graphics-core.
+
+[embedded-graphics]: https://docs.rs/embedded-graphics
+[`Rgb888`]: pixelcolor::Rgb888
+
+## Minimum supported Rust version
+
+The minimum supported Rust version for embedded-graphics-core is `1.71.1` or greater.
+Ensure you have the correct version of Rust installed, preferably through <https://rustup.rs>.
+
+## Development setup
+
+Please see the [development setup guide](../doc/development-setup.md).
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the
+work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
+additional terms or conditions.