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/byte-slice-cast/README.md

@@ -0,0 +1,16 @@
+# byte-slice-cast [![crates.io](https://img.shields.io/crates/v/byte-slice-cast.svg)](https://crates.io/crates/byte-slice-cast) [![Actions Status](https://github.com/sdroege/byte-slice-cast/workflows/byte-slice-cast/badge.svg)](https://github.com/sdroege/byte-slice-cast/actions) [![docs.rs](https://docs.rs/byte-slice-cast/badge.svg)](https://docs.rs/byte-slice-cast)
+
+Safely cast between byte slices and slices of another built-in fundamental number type.
+
+## LICENSE
+
+byte-slice-cast is licensed under the MIT license ([LICENSE](LICENSE) or
+http://opensource.org/licenses/MIT).
+
+## Contribution
+
+Any kinds of contributions are welcome as a pull request.
+
+Unless you explicitly state otherwise, any contribution intentionally
+submitted for inclusion in byte-slice-cast by you shall be licensed under the
+MIT license as above, without any additional terms or conditions.