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/embedded-graphics/release.toml

@@ -0,0 +1,10 @@
+pre-release-replacements = [
+  { file = "CHANGELOG.md", prerelease = true, search = "[Uu]nreleased", replace = "{{version}}" },
+  { file = "CHANGELOG.md", prerelease = true, search = "\\.\\.\\.HEAD", replace = "...{{tag_name}}" },
+  { file = "CHANGELOG.md", prerelease = true, search = "ReleaseDate", replace = "{{date}}" },
+  { file = "CHANGELOG.md", prerelease = true, search = "<!-- next-header -->", replace = "<!-- next-header -->\n\n## [Unreleased] - ReleaseDate" },
+  { file = "CHANGELOG.md", prerelease = true, search = "<!-- next-url -->", replace = "<!-- next-url -->\n[unreleased]: https://github.com/embedded-graphics/{{crate_name}}/compare/{{tag_name}}...HEAD" },
+]
+tag-message = "Release {{crate_name}} {{version}}"
+tag-prefix = "{{crate_name}}-"
+pre-release-commit-message = "Release {{crate_name}} {{version}}"