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/fixed/etc/index.html

@@ -0,0 +1,17 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <title>Fixed-point numbers</title>
+  </head>
+  <body>
+    <div style="margin-right: auto; margin-left: auto; max-width: 600px;">
+      <h1>Fixed-point numbers</h1>
+      <p>The <a href="https://crates.io/crates/fixed"><i>fixed</i> crate</a> provides fixed-point numbers. Here you can find the documentation for the <a href="https://gitlab.com/tspiteri/fixed">current development version</a> of the crate.</p>
+      <h2>Documentation</h2>
+      <ul>
+	<li><a href="dev/fixed/index.html">Documentation for the current development version.</a></li>
+	<li><a href="https://docs.rs/fixed">Documentation for the published versions.</a></li>
+      </ul>
+    </div>
+  </body>
+</html>