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-io-async/CHANGELOG.md

@@ -0,0 +1,28 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## Unreleased
+
+Add unreleased changes here
+
+## 0.6.1 - 2023-11-28
+
+- Use `feature()` on nightly toolchains only. This adds support for 1.75 beta and stable.
+
+## 0.6.0 - 2023-10-02
+
+- Prohibit `Write::write` implementations returning `Ok(0)` unless there is no
+  data to write; consequently remove `WriteAllError`.
+  Update the `&mut [u8]` impl to possibly return
+  a new `SliceWriteError` if the slice is full instead of `Ok(0)`.
+- Add `WriteZero` variant to `ErrorKind` for implementations that previously
+  may have returned `Ok(0)` to indicate no further data could be written.
+- `Write::write_all` now panics if the `write()` implementation returns `Ok(0)`.
+
+## 0.5.0 - 2023-08-06
+
+- First release