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/basic-toml/examples/decode.rs

@@ -0,0 +1,54 @@
+//! An example showing off the usage of `Deserialize` to automatically decode
+//! TOML into a Rust `struct`
+
+#![deny(warnings)]
+#![allow(dead_code)]
+
+use serde_derive::Deserialize;
+
+/// This is what we're going to decode into. Each field is optional, meaning
+/// that it doesn't have to be present in TOML.
+#[derive(Debug, Deserialize)]
+struct Config {
+    global_string: Option<String>,
+    global_integer: Option<u64>,
+    server: Option<ServerConfig>,
+    peers: Option<Vec<PeerConfig>>,
+}
+
+/// Sub-structs are decoded from tables, so this will decode from the `[server]`
+/// table.
+///
+/// Again, each field is optional, meaning they don't have to be present.
+#[derive(Debug, Deserialize)]
+struct ServerConfig {
+    ip: Option<String>,
+    port: Option<u64>,
+}
+
+#[derive(Debug, Deserialize)]
+struct PeerConfig {
+    ip: Option<String>,
+    port: Option<u64>,
+}
+
+fn main() {
+    let toml_str = r#"
+        global_string = "test"
+        global_integer = 5
+
+        [server]
+        ip = "127.0.0.1"
+        port = 80
+
+        [[peers]]
+        ip = "127.0.0.1"
+        port = 8080
+
+        [[peers]]
+        ip = "127.0.0.1"
+    "#;
+
+    let decoded: Config = basic_toml::from_str(toml_str).unwrap();
+    println!("{:#?}", decoded);
+}