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/strsim/SECURITY.md

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+## Reporting Security Issues
+
+If you believe you have found a security vulnerability in the project, please report it to us through coordinated disclosure.
+
+**Please do not report security vulnerabilities through public GitHub issues, discussions, or pull requests.**
+
+Instead, please send an email to oss@maxbachmann.de.
+
+Please include as much of the information listed below as you can to help us better understand and resolve the issue:
+
+  * The type of issue (e.g., buffer overflow, SQL injection, or cross-site scripting)
+  * Full paths of source file(s) related to the manifestation of the issue
+  * The location of the affected source code (tag/branch/commit or direct URL)
+  * Any special configuration required to reproduce the issue
+  * Step-by-step instructions to reproduce the issue
+  * Proof-of-concept or exploit code (if possible)
+  * Impact of the issue, including how an attacker might exploit the issue
+
+This information will help us triage your report more quickly.