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/embassy-usb/README.md

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+# embassy-usb
+
+Async USB device stack for embedded devices in Rust.
+
+## Features
+
+- Native async.
+- Fully lock-free: endpoints are separate objects that can be used independently without needing a central mutex. If the driver supports it, they can even be used from different priority levels.
+- Suspend/resume, remote wakeup.
+- USB composite devices.
+- Ergonomic descriptor builder.
+- Ready-to-use implementations for a few USB classes (note you can still implement any class yourself outside the crate).
+    - Serial ports (CDC ACM)
+    - Ethernet (CDC NCM)
+    - Human Interface Devices (HID)
+    - MIDI
+
+## Adding support for new hardware
+
+To add `embassy-usb` support for new hardware (i.e. a new MCU chip), you have to write a driver that implements
+the [`embassy-usb-driver`](https://crates.io/crates/embassy-usb-driver) traits.
+
+Driver crates should depend only on `embassy-usb-driver`, not on the main `embassy-usb` crate.
+This allows existing drivers to continue working for newer `embassy-usb` major versions, without needing an update, if the driver
+trait has not had breaking changes.
+
+## Configuration
+
+`embassy-usb` has some configuration settings that are set at compile time, affecting sizes
+and counts of buffers.
+
+They can be set in two ways:
+
+- Via Cargo features: enable a feature like `<name>-<value>`. `name` must be in lowercase and
+use dashes instead of underscores. For example. `max-interface-count-3`. Only a selection of values
+is available, check `Cargo.toml` for the list.
+- Via environment variables at build time: set the variable named `EMBASSY_USB_<value>`. For example
+`EMBASSY_USB_MAX_INTERFACE_COUNT=3 cargo build`. You can also set them in the `[env]` section of `.cargo/config.toml`.
+Any value can be set, unlike with Cargo features.
+
+Environment variables take precedence over Cargo features. If two Cargo features are enabled for the same setting
+with different values, compilation fails.
+
+### `MAX_INTERFACE_COUNT`
+
+Max amount of interfaces that can be created in one device. Default: 4.
+
+## Interoperability
+
+This crate can run on any executor.