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/windows-sys/src/lib.rs

@@ -0,0 +1,17 @@
+/*!
+Learn more about Rust for Windows here: <https://github.com/microsoft/windows-rs>
+
+[Feature search](https://microsoft.github.io/windows-rs/features/#/0.59.0)
+*/
+
+#![no_std]
+#![doc(html_no_source)]
+#![allow(non_snake_case, non_upper_case_globals, non_camel_case_types, missing_docs, clippy::all)]
+#![cfg_attr(not(feature = "docs"), doc(hidden))]
+
+#[allow(unused_extern_crates)]
+extern crate self as windows_sys;
+
+pub mod core;
+
+include!("Windows/mod.rs");