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/paste/tests/ui/unsupported-literal.rs

@@ -0,0 +1,21 @@
+use paste::paste;
+
+paste! {
+    fn [<x 1e+100 z>]() {}
+}
+
+paste! {
+    // `xyz` is not correct. `xbyz` is certainly not correct. Maybe `x121z`
+    // would be justifiable but for now don't accept this.
+    fn [<x b'y' z>]() {}
+}
+
+paste! {
+    fn [<x b"y" z>]() {}
+}
+
+paste! {
+    fn [<x br"y" z>]() {}
+}
+
+fn main() {}