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/macro_free.rs

@@ -0,0 +1,61 @@
+//! An example of implementing the `BitFlags` trait manually for a flags type.
+//!
+//! This example doesn't use any macros.
+
+use std::{fmt, str};
+
+use bitflags::{Flag, Flags};
+
+// First: Define your flags type. It just needs to be `Sized + 'static`.
+pub struct ManualFlags(u32);
+
+// Not required: Define some constants for valid flags
+impl ManualFlags {
+    pub const A: ManualFlags = ManualFlags(0b00000001);
+    pub const B: ManualFlags = ManualFlags(0b00000010);
+    pub const C: ManualFlags = ManualFlags(0b00000100);
+    pub const ABC: ManualFlags = ManualFlags(0b00000111);
+}
+
+// Next: Implement the `BitFlags` trait, specifying your set of valid flags
+// and iterators
+impl Flags for ManualFlags {
+    const FLAGS: &'static [Flag<Self>] = &[
+        Flag::new("A", Self::A),
+        Flag::new("B", Self::B),
+        Flag::new("C", Self::C),
+    ];
+
+    type Bits = u32;
+
+    fn bits(&self) -> u32 {
+        self.0
+    }
+
+    fn from_bits_retain(bits: u32) -> Self {
+        Self(bits)
+    }
+}
+
+// Not required: Add parsing support
+impl str::FromStr for ManualFlags {
+    type Err = bitflags::parser::ParseError;
+
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
+        bitflags::parser::from_str(input)
+    }
+}
+
+// Not required: Add formatting support
+impl fmt::Display for ManualFlags {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        bitflags::parser::to_writer(self, f)
+    }
+}
+
+fn main() {
+    println!(
+        "{}",
+        ManualFlags::A.union(ManualFlags::B).union(ManualFlags::C)
+    );
+}