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/bitflags/examples/serde.rs

@@ -0,0 +1,39 @@
+//! An example of implementing `serde::Serialize` and `serde::Deserialize`.
+//! The `#[serde(transparent)]` attribute is recommended to serialize directly
+//! to the underlying bits type without wrapping it in a `serde` newtype.
+
+#[cfg(feature = "serde")]
+fn main() {
+    use serde_lib::*;
+
+    bitflags::bitflags! {
+        #[derive(Serialize, Deserialize, Debug, PartialEq, Eq)]
+        #[serde(transparent)]
+        // NOTE: We alias the `serde` crate as `serde_lib` in this repository,
+        // but you don't need to do this
+        #[serde(crate = "serde_lib")]
+        pub struct Flags: u32 {
+            const A = 1;
+            const B = 2;
+            const C = 4;
+            const D = 8;
+        }
+    }
+
+    let flags = Flags::A | Flags::B;
+
+    let serialized = serde_json::to_string(&flags).unwrap();
+
+    println!("{:?} -> {}", flags, serialized);
+
+    assert_eq!(serialized, r#""A | B""#);
+
+    let deserialized: Flags = serde_json::from_str(&serialized).unwrap();
+
+    println!("{} -> {:?}", serialized, flags);
+
+    assert_eq!(deserialized, flags);
+}
+
+#[cfg(not(feature = "serde"))]
+fn main() {}