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/ssd1306/.github/PULL_REQUEST_TEMPLATE.md

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+Hi! Thank you for helping out with SSD1306 development! Please:
+
+- [ ] Check that you've added documentation to any new methods
+- [ ] Rebase from `master` if you're not already up to date
+- [ ] Add or modify an example if there are changes to the public API
+- [ ] Add a `CHANGELOG.md` entry in the **Unreleased** section under the appropriate heading (**Added**, **Fixed**, **Changed**, etc)
+- [ ] Run `rustfmt` on the project with `cargo fmt --all` - CI will not pass without this step
+- [ ] Check that your branch is up to date with master and that CI is passing once the PR is opened
+
+## PR description
+
+[add your PR description here]