feat: show MAC address on splash screen

Splash/waiting screen now shows:
  Row 1: "DongLoRa" (bold)
  Row 2: "Board Name vX.Y.Z"
  Row 3: MAC address (XX:XX:XX:XX:XX:XX)
  Row 4: "Waiting for host..."

Each board reads its MAC at init:
- ESP32-S3: efuse MAC via esp_hal::efuse::Efuse::mac_address()
- nRF52840: FICR device address registers

BoardInfo struct bundles name/version/mac to keep dashboard() under
clippy's argument limit.

src/board/heltec_v3.rs

@@ -41,6 +41,7 @@ pub struct UsbParts {
 
 pub struct DisplayParts {
     pub i2c: DisplayI2c,
+    pub mac: [u8; 6],
 }
 
 // ── Board init ───────────────────────────────────────────────────────
@@ -130,7 +131,8 @@ impl Board {
             .with_sda(p.GPIO17)
             .with_scl(p.GPIO18)
             .into_async();
-        let display = Some(DisplayParts { i2c });
+        let mac = esp_hal::efuse::Efuse::mac_address();
+        let display = Some(DisplayParts { i2c, mac });
 
         (radio, usb, display)
     }