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/thiserror/tests/ui/raw-identifier.stderr

@@ -0,0 +1,21 @@
+error: invalid format string: raw identifiers are not supported
+ --> tests/ui/raw-identifier.rs:4:18
+  |
+4 | #[error("error: {r#fn}")]
+  |                  --^^
+  |                  |
+  |                  raw identifier used here in format string
+  |                  help: remove the `r#`
+  |
+  = note: identifiers in format strings can be keywords and don't need to be prefixed with `r#`
+
+error: invalid format string: raw identifiers are not supported
+  --> tests/ui/raw-identifier.rs:11:30
+   |
+11 |     let _ = format!("error: {r#fn}");
+   |                              --^^
+   |                              |
+   |                              raw identifier used here in format string
+   |                              help: remove the `r#`
+   |
+   = note: identifiers in format strings can be keywords and don't need to be prefixed with `r#`