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/embedded-storage/README.md

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+# embedded-storage
+
+> An Embedded Storage Abstraction Layer
+
+This crate defines a set of traits that can be implemented to provide
+non-volatile storage of different kinds, including but not limited to EEPROM,
+NOR-flash & NAND-flash, both external and internal.
+
+## [API reference]
+
+## How-to: add a new trait
+
+This is the suggested approach to adding a new trait to `embedded-storage`
+
+### Research / Discussion
+
+Ideally, before proposing a new trait, or set of traits, you should check for an existing issue
+suggesting the need for the trait, as well as any related works / use cases / requirements that
+are useful to consider in the design of the trait.
+
+These issues will be labeled as `discussion` in the issue tracker.
+
+### Implementation / Demonstration
+
+Proposed traits should then be implemented and demonstrated, either by forking `embedded-storage` or by creating a new crate with the intent of integrating this into `embedded-storage` once the traits have stabilized. You may find [cargo workspaces](https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html) and [patch](https://doc.rust-lang.org/edition-guide/rust-2018/cargo-and-crates-io/replacing-dependencies-with-patch.html) useful for the forking approach.
+
+### Proposing a trait
+
+Once the trait has been demonstrated a PR should be opened to merge the new trait(s) into `embedded-storage`. This should include a link to the previous discussion issue.
+
+If there is determined to be more than one alternative then there should be further discussion to
+try to single out the best option. Once there is consensus this will be merged into the `embedded-storage` repository.
+
+These issues / PRs will be labeled as `proposal`s in the issue tracker.
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.50.0 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
+## License
+
+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.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+[API reference]: https://docs.rs/embedded-storage