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-embedded-hal/README.md

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+# embassy-embedded-hal
+
+Collection of utilities to use `embedded-hal` and `embedded-storage` traits with Embassy.
+
+- Shared SPI and I2C buses, both blocking and async, with a `SetConfig` trait allowing changing bus configuration (e.g. frequency) between devices on the same bus.
+- Async utilities
+    - Adapters to convert from blocking to (fake) async.
+    - Adapters to insert yields on trait operations.
+- Flash utilities
+    - Split a flash memory into smaller partitions.
+    - Concatenate flash memories together.
+    - Simulated in-memory flash.