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-time-queue-driver/README.md

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+# embassy-time-queue-driver
+
+This crate contains the driver trait used by the [`embassy-time`](https://crates.io/crates/embassy-time) timer queue.
+
+You should rarely need to use this crate directly. Only use it when implementing your own timer queue.
+
+There is two timer queue implementations, one in `embassy-time` enabled by the `generic-queue` feature, and 
+another in `embassy-executor` enabled by the `integrated-timers` feature.