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/fmt.rs

@@ -0,0 +1,49 @@
+//! An example of implementing Rust's standard formatting and parsing traits for flags types.
+
+use core::{fmt, str};
+
+bitflags::bitflags! {
+    // You can `#[derive]` the `Debug` trait, but implementing it manually
+    // can produce output like `A | B` instead of `Flags(A | B)`.
+    // #[derive(Debug)]
+    #[derive(PartialEq, Eq)]
+    pub struct Flags: u32 {
+        const A = 1;
+        const B = 2;
+        const C = 4;
+        const D = 8;
+    }
+}
+
+impl fmt::Debug for Flags {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        bitflags::parser::to_writer(self, f)
+    }
+}
+
+impl fmt::Display for Flags {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        bitflags::parser::to_writer(self, f)
+    }
+}
+
+impl str::FromStr for Flags {
+    type Err = bitflags::parser::ParseError;
+
+    fn from_str(flags: &str) -> Result<Self, Self::Err> {
+        bitflags::parser::from_str(flags)
+    }
+}
+
+fn main() -> Result<(), bitflags::parser::ParseError> {
+    let flags = Flags::A | Flags::B;
+
+    println!("{}", flags);
+
+    let formatted = flags.to_string();
+    let parsed: Flags = formatted.parse()?;
+
+    assert_eq!(flags, parsed);
+
+    Ok(())
+}