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/cfg-if/README.md

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+# cfg-if
+
+[Documentation](https://docs.rs/cfg-if)
+
+A macro to ergonomically define an item depending on a large number of #[cfg]
+parameters. Structured like an if-else chain, the first matching branch is the
+item that gets emitted.
+
+```toml
+[dependencies]
+cfg-if = "1.0"
+```
+
+## Example
+
+```rust
+cfg_if::cfg_if! {
+    if #[cfg(unix)] {
+        fn foo() { /* unix specific functionality */ }
+    } else if #[cfg(target_pointer_width = "32")] {
+        fn foo() { /* non-unix, 32-bit functionality */ }
+    } else {
+        fn foo() { /* fallback implementation */ }
+    }
+}
+
+fn main() {
+    foo();
+}
+```
+The `cfg_if!` block above is expanded to:
+```rust
+#[cfg(unix)]
+fn foo() { /* unix specific functionality */ }
+#[cfg(all(target_pointer_width = "32", not(unix)))]
+fn foo() { /* non-unix, 32-bit functionality */ }
+#[cfg(not(any(unix, target_pointer_width = "32")))]
+fn foo() { /* fallback implementation */ }
+```
+
+# License
+
+This project is licensed under either of
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+   https://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+   https://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in `cfg-if` by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.