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/bitflags-1.3.2/README.md

@@ -0,0 +1,32 @@
+bitflags
+========
+
+[![Rust](https://github.com/bitflags/bitflags/workflows/Rust/badge.svg)](https://github.com/bitflags/bitflags/actions)
+[![Join the chat at https://gitter.im/bitflags/Lobby](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/bitflags/Lobby?utm_source=badge&utm_medium=badge&utm_content=badge)
+[![Latest version](https://img.shields.io/crates/v/bitflags.svg)](https://crates.io/crates/bitflags)
+[![Documentation](https://docs.rs/bitflags/badge.svg)](https://docs.rs/bitflags)
+![License](https://img.shields.io/crates/l/bitflags.svg)
+
+A Rust macro to generate structures which behave like a set of bitflags
+
+- [Documentation](https://docs.rs/bitflags)
+- [Release notes](https://github.com/bitflags/bitflags/releases)
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+bitflags = "1.3"
+```
+
+and this to your source code:
+
+```rust
+use bitflags::bitflags;
+```
+
+## Rust Version Support
+
+The minimum supported Rust version is 1.46 due to use of associated constants and const functions.