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/futures-util/src/never.rs

@@ -0,0 +1,18 @@
+//! This module contains the `Never` type.
+//!
+//! Values of this type can never be created and will never exist.
+
+/// A type with no possible values.
+///
+/// This is used to indicate values which can never be created, such as the
+/// error type of infallible futures.
+///
+/// This type is a stable equivalent to the `!` type from `std`.
+///
+/// This is currently an alias for [`std::convert::Infallible`], but in
+/// the future it may be an alias for [`!`][never].
+/// See ["Future compatibility" section of `std::convert::Infallible`][infallible] for more.
+///
+/// [never]: https://doc.rust-lang.org/nightly/std/primitive.never.html
+/// [infallible]: https://doc.rust-lang.org/nightly/std/convert/enum.Infallible.html#future-compatibility
+pub type Never = core::convert::Infallible;