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/bytemuck/.cargo/config.toml

@@ -0,0 +1,4 @@
+[alias]
+
+# The list of features should be the same as the one under `[package.metadata.docs.rs]`
+nightly_docs = "doc --no-deps -F nightly_docs,latest_stable_rust,extern_crate_alloc,extern_crate_std"