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/darling/tests/compile-fail/flatten_meta_conflicts.rs

@@ -0,0 +1,21 @@
+use darling::FromMeta;
+
+#[derive(FromMeta)]
+struct Inner {
+    left: String,
+    right: String,
+}
+
+#[derive(FromMeta)]
+struct Outer {
+    #[darling(flatten, multiple, with = demo, skip = true)]
+    field: Inner,
+}
+
+#[derive(FromMeta)]
+struct ThisIsFine {
+    #[darling(flatten, multiple = false)]
+    field: Inner,
+}
+
+fn main() {}