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/custom_derive.rs

@@ -0,0 +1,23 @@
+//! An example of implementing the `BitFlags` trait manually for a flags type.
+
+use std::str;
+
+use bitflags::bitflags;
+
+// Define a flags type outside of the `bitflags` macro as a newtype
+// It can accept custom derives for libraries `bitflags` doesn't support natively
+#[derive(zerocopy::IntoBytes, zerocopy::FromBytes, zerocopy::KnownLayout, zerocopy::Immutable)]
+#[repr(transparent)]
+pub struct ManualFlags(u32);
+
+// Next: use `impl Flags` instead of `struct Flags`
+bitflags! {
+    impl ManualFlags: u32 {
+        const A = 0b00000001;
+        const B = 0b00000010;
+        const C = 0b00000100;
+        const ABC = Self::A.bits() | Self::B.bits() | Self::C.bits();
+    }
+}
+
+fn main() {}