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-sync/README.md

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+# embassy-sync
+
+An [Embassy](https://embassy.dev) project.
+
+Synchronization primitives and data structures with async support:
+
+- [`Channel`](channel::Channel) - A Multiple Producer Multiple Consumer (MPMC) channel. Each message is only received by a single consumer.
+- [`PriorityChannel`](priority_channel::PriorityChannel) - A Multiple Producer Multiple Consumer (MPMC) channel. Each message is only received by a single consumer. Higher priority items are shifted to the front of the channel.
+- [`PubSubChannel`](pubsub::PubSubChannel) - A broadcast channel (publish-subscribe) channel. Each message is received by all consumers.
+- [`Signal`](signal::Signal) - Signalling latest value to a single consumer.
+- [`Watch`](watch::Watch) - Signalling latest value to multiple consumers.
+- [`Mutex`](mutex::Mutex) - Mutex for synchronizing state between asynchronous tasks.
+- [`Pipe`](pipe::Pipe) - Byte stream implementing `embedded_io` traits.
+- [`WakerRegistration`](waitqueue::WakerRegistration) - Utility to register and wake a `Waker`.
+- [`AtomicWaker`](waitqueue::AtomicWaker) - Utility to register and wake a `Waker` from interrupt context.
+- [`MultiWakerRegistration`](waitqueue::MultiWakerRegistration) - Utility registering and waking multiple `Waker`'s.
+- [`LazyLock`](lazy_lock::LazyLock) - A value which is initialized on the first access
+
+## Interoperability
+
+Futures from this crate can run on any executor.