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/serde/src/integer128.rs

@@ -0,0 +1,14 @@
+#[macro_export]
+#[deprecated = "
+This macro has no effect on any version of Serde released in the past 2 years.
+It was used long ago in crates that needed to support Rustc older than 1.26.0,
+or Emscripten targets older than 1.40.0, which did not yet have 128-bit integer
+support. These days Serde requires a Rust compiler newer than that so 128-bit
+integers are always supported.
+"]
+#[doc(hidden)]
+macro_rules! serde_if_integer128 {
+    ($($tt:tt)*) => {
+        $($tt)*
+    };
+}