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/expression-fallback.stderr

@@ -0,0 +1,24 @@
+error: expected `,`, found `.`
+ --> tests/ui/expression-fallback.rs:4:11
+  |
+4 | #[error("".yellow)]
+  |           ^ expected `,`
+
+error: argument never used
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |         -- ^^^^^^ argument never used
+  |         |
+  |         formatting specifier missing
+  |
+help: format specifiers use curly braces, consider adding a format specifier
+  |
+4 | #[error("{}".yellow)]
+  |          ++
+
+error[E0425]: cannot find value `yellow` in this scope
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |            ^^^^^^ not found in this scope