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/unicase/README.md

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+# unicase
+
+[![crates.io](https://img.shields.io/crates/v/unicase.svg)](https://crates.io/crates/unicase)
+[![Released API docs](https://docs.rs/unicase/badge.svg)](https://docs.rs/unicase)
+[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)
+[![CI](https://github.com/seanmonstar/unicase/workflows/CI/badge.svg)](https://github.com/seanmonstar/unicase/actions?query=workflow%3ACI)
+
+Compare strings when case is not important (using Unicode Case-folding).
+
+```rust
+// ignore ASCII case
+let a = UniCase::new("foobar");
+let b = UniCase::new("FOOBAR");
+
+assert_eq!(a, b);
+
+// using unicode case-folding
+let c = UniCase::new("Maße")
+let d = UniCase::new("MASSE");
+assert_eq!(c, d);
+```
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.