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/toml/examples/enum_external.rs

@@ -0,0 +1,45 @@
+//! An example showing off the usage of `Deserialize` to automatically decode
+//! TOML into a Rust `struct`, with enums.
+
+#![deny(warnings)]
+#![allow(dead_code)]
+
+use serde::Deserialize;
+
+/// This is what we're going to decode into.
+#[derive(Debug, Deserialize)]
+struct Config {
+    plain: MyEnum,
+    plain_table: MyEnum,
+    tuple: MyEnum,
+    #[serde(rename = "struct")]
+    structv: MyEnum,
+    newtype: MyEnum,
+    my_enum: Vec<MyEnum>,
+}
+
+#[derive(Debug, Deserialize)]
+enum MyEnum {
+    Plain,
+    Tuple(i64, bool),
+    NewType(String),
+    Struct { value: i64 },
+}
+
+fn main() {
+    let toml_str = r#"
+    plain = "Plain"
+    plain_table = { Plain = {} }
+    tuple = { Tuple = { 0 = 123, 1 = true } }
+    struct = { Struct = { value = 123 } }
+    newtype = { NewType = "value" }
+    my_enum = [
+        { Plain = {} },
+        { Tuple = { 0 = 123, 1 = true } },
+        { NewType = "value" },
+        { Struct = { value = 123 } }
+    ]"#;
+
+    let decoded: Config = toml::from_str(toml_str).unwrap();
+    println!("{decoded:#?}");
+}