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/winapi-util/src/lib.rs

@@ -0,0 +1,35 @@
+/*!
+This crate provides a smattering of safe routines for parts of windows-sys. The
+primary purpose of this crate is to serve as a dumping ground for various
+utility functions that make interactions with windows-sys safe. This permits the
+centralization of `unsafe` when dealing with Windows APIs, and thus makes it
+easier to audit.
+
+A key abstraction in this crate is the combination of the
+[`Handle`](struct.Handle.html)
+and
+[`HandleRef`](struct.HandleRef.html)
+types. Both represent a valid Windows handle to an I/O-like object, where
+`Handle` is owned (the resource is closed when the handle is dropped) and
+`HandleRef` is borrowed (the resource is not closed when the handle is
+dropped). Many of the routines in this crate work on handles and accept
+anything that can be safely converted into a `HandleRef`. This includes
+standard library types such as `File`, `Stdin`, `Stdout` and `Stderr`.
+
+Note that this crate is completely empty on non-Windows platforms.
+*/
+
+#[cfg(windows)]
+pub use win::*;
+
+/// Safe routines for dealing with the Windows console.
+#[cfg(windows)]
+pub mod console;
+/// Safe routines for dealing with files and handles on Windows.
+#[cfg(windows)]
+pub mod file;
+#[cfg(windows)]
+/// Safe routines for querying various Windows specific properties.
+pub mod sysinfo;
+#[cfg(windows)]
+mod win;