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/esp-hal/src/prelude.rs

@@ -0,0 +1,43 @@
+//! # The `esp-hal` Prelude
+//!
+//! ## Overview
+//! The prelude is the list of things that `esp-hal` automatically imports into
+//! every program. It’s kept as small as possible, and is focused on
+//! things, particularly traits, which are used in almost every single Rust
+//! program.
+
+pub use imp::*;
+
+#[doc(hidden)]
+mod imp {
+    #[doc(hidden)]
+    pub use fugit::{ExtU64 as _, RateExtU32 as _};
+    #[doc(hidden)]
+    pub use nb;
+
+    #[cfg(dac)]
+    pub use crate::analog::dac::Instance as _esp_hal_analog_dac_Instance;
+    #[cfg(any(dport, pcr, system))]
+    pub use crate::clock::Clock as _esp_hal_clock_Clock;
+    #[cfg(gpio)]
+    pub use crate::gpio::Pin as _esp_hal_gpio_Pin;
+    #[cfg(ledc)]
+    pub use crate::ledc::{
+        channel::{
+            ChannelHW as _esp_hal_ledc_channel_ChannelHW,
+            ChannelIFace as _esp_hal_ledc_channel_ChannelIFace,
+        },
+        timer::{
+            TimerHW as _esp_hal_ledc_timer_TimerHW,
+            TimerIFace as _esp_hal_ledc_timer_TimerIFace,
+        },
+    };
+    #[cfg(any(timg0, timg1))]
+    pub use crate::timer::timg::{
+        Instance as _esp_hal_timer_timg_Instance,
+        TimerGroupInstance as _esp_hal_timer_timg_TimerGroupInstance,
+    };
+    #[cfg(any(systimer, timg0, timg1))]
+    pub use crate::timer::Timer as _esp_hal_timer_Timer;
+    pub use crate::{clock::CpuClock, entry, macros::*, InterruptConfigurable};
+}