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/embassy-sync-0.5.0/CHANGELOG.md

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+# 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).
+
+## 0.5.0 - 2023-12-04
+
+- Add a PriorityChannel.
+- Remove nightly and unstable-traits features in preparation for 1.75.
+- Upgrade heapless to 0.8.
+- Upgrade static-cell to 2.0.
+
+## 0.4.0 - 2023-10-31
+
+- Re-add impl_trait_projections
+- switch to `embedded-io 0.6`
+
+## 0.3.0 - 2023-09-14
+
+- switch to `embedded-io 0.5`
+- add api for polling channels with context
+- standardise fn names on channels
+- add zero-copy channel
+
+## 0.2.0 - 2023-04-13
+
+- pubsub: Fix messages not getting popped when the last subscriber that needed them gets dropped.
+- pubsub: Move instead of clone messages when the last subscriber pops them.
+- pubsub: Pop messages which count is 0 after unsubscribe.
+- Update `embedded-io` from `0.3` to `0.4` (uses `async fn` in traits).
+- impl `Default` for `WakerRegistration`
+- impl `Default` for `Signal`
+- Remove unnecessary uses of `atomic-polyfill`
+- Add `#[must_use]` to all futures.
+
+
+## 0.1.0 - 2022-08-26
+
+- First release