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/esp-hal/ld/README.md

@@ -0,0 +1,8 @@
+# ROM functions
+
+Files in the `rom` subdirectories are taken from esp-idf
+
+- DON'T include any `*newlib*` functions
+- systimer, wdt and mbedtls shouldn't be included
+- make sure to align the version you take the files from with esp-wifi-sys - NEVER randomly sync the files with other versions
+- some additional functions are needed from ROM - see `additional.ld` (these are usually defined in the `*newlib*` files)