Combined flash + key exchange flow with version check and USB permission persistence

- Firmware: add VERSION? command returning BUILD_NR for version check
- Android: flash button checks firmware version first, skips flash if current
- After flash: auto key exchange via reboot detection (expectingReboot flag)
- device_filter.xml + intent-filter: USB permission 'always allow' checkbox
- singleTop + onNewIntent prevents unwanted SettingsActivity launch on USB connect
- platformio.ini: upload_port updated to /dev/ttyACM1

src/main.cpp

@@ -22,6 +22,7 @@
 #define NVS_NAMESPACE "hmac"
 #define NVS_KEY_NAME  "key"
 #define NVS_UUID_NAME "uuid"
+#define FIRMWARE_VERSION "1"
 
 NimBLECharacteristic *pCharacteristic;
 bool deviceConnected = false;
@@ -250,6 +251,9 @@ void loop() {
                 } else {
                     Serial.println("NO_KEY");
                 }
+            } else if (serialBuf == "VERSION?") {
+                Serial.print("BUILD_NR:");
+                Serial.println(FIRMWARE_VERSION);
             } else if (serialBuf.startsWith("SET_KEY:")) {
                 String hex = serialBuf.substring(8);
                 hex.toUpperCase();