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-link/src/lib.rs

@@ -0,0 +1,39 @@
+#![doc = include_str!("../readme.md")]
+#![no_std]
+
+/// Defines an external function to import.
+#[cfg(all(windows, target_arch = "x86"))]
+#[macro_export]
+macro_rules! link {
+    ($library:literal $abi:literal $($link_name:literal)? fn $($function:tt)*) => (
+        #[link(name = $library, kind = "raw-dylib", modifiers = "+verbatim", import_name_type = "undecorated")]
+        extern $abi {
+            $(#[link_name=$link_name])?
+            pub fn $($function)*;
+        }
+    )
+}
+
+/// Defines an external function to import.
+#[cfg(all(windows, not(target_arch = "x86")))]
+#[macro_export]
+macro_rules! link {
+    ($library:literal $abi:literal $($link_name:literal)? fn $($function:tt)*) => (
+        #[link(name = $library, kind = "raw-dylib", modifiers = "+verbatim")]
+        extern $abi {
+            $(#[link_name=$link_name])?
+            pub fn $($function)*;
+        }
+    )
+}
+
+/// Defines an external function to import.
+#[cfg(not(windows))]
+#[macro_export]
+macro_rules! link {
+    ($library:literal $abi:literal $($link_name:literal)? fn $($function:tt)*) => (
+        extern $abi {
+            pub fn $($function)*;
+        }
+    )
+}