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/toml2json.rs

@@ -0,0 +1,43 @@
+use std::env;
+use std::io;
+use std::io::prelude::*;
+
+use serde_json::Value as Json;
+use toml::Value as Toml;
+
+fn main() {
+    let mut args = env::args();
+    let input = if args.len() > 1 {
+        let name = args.nth(1).unwrap();
+        std::fs::read_to_string(name).unwrap()
+    } else {
+        let mut input = String::new();
+        io::stdin().read_to_string(&mut input).unwrap();
+        input
+    };
+
+    match input.parse() {
+        Ok(toml) => {
+            let json = convert(toml);
+            println!("{}", serde_json::to_string_pretty(&json).unwrap());
+        }
+        Err(error) => println!("failed to parse TOML: {error}"),
+    }
+}
+
+fn convert(toml: Toml) -> Json {
+    match toml {
+        Toml::String(s) => Json::String(s),
+        Toml::Integer(i) => Json::Number(i.into()),
+        Toml::Float(f) => {
+            let n = serde_json::Number::from_f64(f).expect("float infinite and nan not allowed");
+            Json::Number(n)
+        }
+        Toml::Boolean(b) => Json::Bool(b),
+        Toml::Array(arr) => Json::Array(arr.into_iter().map(convert).collect()),
+        Toml::Table(table) => {
+            Json::Object(table.into_iter().map(|(k, v)| (k, convert(v))).collect())
+        }
+        Toml::Datetime(dt) => Json::String(dt.to_string()),
+    }
+}