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-net-driver/README.md

@@ -0,0 +1,28 @@
+# embassy-net-driver
+
+This crate contains the driver trait necessary for adding [`embassy-net`](https://crates.io/crates/embassy-net) support
+for a new hardware platform.
+
+If you want to *use* `embassy-net` with already made drivers, you should depend on the main `embassy-net` crate, not on this crate.
+
+If you are writing a driver, you  should depend only on this crate, not on the main `embassy-net` crate.
+This will allow your driver to continue working for newer `embassy-net` major versions, without needing an update,
+if the driver trait has not had breaking changes.
+
+See also [`embassy-net-driver-channel`](https://crates.io/crates/embassy-net-driver-channel), which provides a higer-level API
+to construct a driver that processes packets in its own background task and communicates with the `embassy-net` task via
+packet queues for RX and TX.
+
+## Interoperability
+
+This crate can run on any executor.
+
+## License
+
+This work is licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.