refactor: rename rak_4631 to rak_wisblock_4631, add proper display names

Board naming now follows MeshCore conventions:
- rak_4631 → rak_wisblock_4631 (feature, file, Justfile, splash screen)
- Display splash shows human-readable names: "RAK WisBlock 4631",
  "Heltec v3", "Heltec v4"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -54,7 +54,7 @@ heltec_v4 = [
     "dep:esp-hal-embassy",
     "dep:esp-println",
 ]
-rak_4631 = [
+rak_wisblock_4631 = [
     "dep:cortex-m-rt",
     "dep:embassy-nrf",
     "dep:panic-probe",