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/pulldown-cmark/examples/string-to-string.rs

@@ -0,0 +1,24 @@
+use pulldown_cmark::{html, Options, Parser};
+
+fn main() {
+    let markdown_input: &str = "Hello world, this is a ~~complicated~~ *very simple* example.";
+    println!("Parsing the following markdown string:\n{}", markdown_input);
+
+    // Set up options and parser. Strikethroughs are not part of the CommonMark standard
+    // and we therefore must enable it explicitly.
+    let mut options = Options::empty();
+    options.insert(Options::ENABLE_STRIKETHROUGH);
+    let parser = Parser::new_ext(markdown_input, options);
+
+    // Write to String buffer.
+    let mut html_output: String = String::with_capacity(markdown_input.len() * 3 / 2);
+    html::push_html(&mut html_output, parser);
+
+    // Check that the output is what we expected.
+    let expected_html: &str =
+        "<p>Hello world, this is a <del>complicated</del> <em>very simple</em> example.</p>\n";
+    assert_eq!(expected_html, &html_output);
+
+    // Write result to stdout.
+    println!("\nHTML output:\n{}", &html_output);
+}