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/embassy-usb-driver/release.toml

@@ -0,0 +1,5 @@
+pre-release-replacements = [
+    {file="CHANGELOG.md", search="Unreleased", replace="{{version}}", min=1},
+    {file="CHANGELOG.md", search="ReleaseDate", replace="{{date}}", min=1},
+    {file="CHANGELOG.md", search="<!-- next-header -->", replace="<!-- next-header -->\n## Unreleased - ReleaseDate\n", exactly=1},
+]