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/ISSUE_TEMPLATE.md

@@ -0,0 +1,13 @@
+- Version of `ssd1306` in use (if applicable): [version here]
+- MCU/other hardware in use: [hardware description here]
+- Display resolution and interface: [SPI/I2C], [128x64]
+
+## Description of the problem/feature request/other
+
+[description here]
+
+## Test case (if applicable)
+
+```rust
+// Failing test case demonstrating the issue here
+```