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-embassy/MIGRATING-0.3.md

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+# Migration Guide from 0.3.x to v0.4.x
+
+## Initialization
+
+You no longer have to set up clocks and pass them to `esp_hal_embassy::init`.
+
+```diff
+ use esp_hal::{
+-    clock::ClockControl,
+-    peripherals::Peripherals,
+     prelude::*,
+-    system::SystemControl,
+ };
+
+ #[esp_hal_embassy::main]
+ async fn main(_spawner: Spawner) -> ! {
+-    let peripherals = Peripherals::take();
+-    let system = SystemControl::new(peripherals.SYSTEM);
+-    let clocks = ClockControl::boot_defaults(system.clock_control).freeze();
++    let peripherals = esp_hal::init(esp_hal::Config::default());
+
+     let timg0 = TimerGroup::new(peripherals.TIMG0);
+-    esp_hal_embassy::init(&clocks, timg0);
++    esp_hal_embassy::init(timg0.timer0);
+
+     // ...
+ }
+```
+
+You have to specify a timer instance (that may be a `TimerGroup` timer unit
+or a `SystemTimer` alarm) or an array of `AnyTimer`s when calling `init`.
+An example of how you can set multiple timers (for example when using
+multiple executors):
+
+```rust
+use esp_hal::{
+    prelude::*,
+    timer::{
+        AnyTimer,
+        systimer::SystemTimer
+    }
+};
+
+#[esp_hal_embassy::main]
+async fn main(_spawner: Spawner) -> ! {
+    let peripherals = esp_hal::init(esp_hal::Config::default());
+
+    let timg0 = TimerGroup::new(peripherals.TIMG0);
+    let timer0: AnyTimer = timg0.timer0.into();
+    let timer1: AnyTimer = timg0.timer1.into();
+
+    // You can use either a TimerGroup timer, a SystemTimer alarm,
+    // or you can mix and match them as well.
+    let systimer = SystemTimer::new(peripherals.SYSTIMER).split::<Target>();
+    let timer2: AnyTimer = systimer.alarm0;
+
+    esp_hal_embassy::init([timer0, timer1, timer2]);
+
+    // ...
+}
+```
+
+Note that you only have to convert into `AnyTimer` if you want to use multiple timers.