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-can/CHANGELOG.md

@@ -0,0 +1,23 @@
+# Change Log
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/)
+and this project adheres to [Semantic Versioning](http://semver.org/).
+
+## [Unreleased]
+
+...
+
+## [v0.4.1] - 2022-09-28
+
+### Removed
+- Unnecessary `embedded-hal` dependency.
+
+## [v0.4.0] - 2022-09-28
+
+Release of version of the traits extracted from embedded-hal.
+
+[Unreleased]: https://github.com/rust-embedded/embedded-hal/compare/embedded-can-v0.4.1...HEAD
+[v0.4.1]: https://github.com/rust-embedded/embedded-hal/compare/embedded-can-v0.4.0...embedded-can-v0.4.1
+[v0.4.0]: https://github.com/rust-embedded/embedded-hal/tree/embedded-can-v0.4.0