fix: relay pins G1/G2, OTA CCCD+retry, single version source

- Relay pins: G39→G1, G38→G2
- BUNDLED_FIRMWARE_VERSION → SylDaVersions.SERVER (version.txt)
- OTA: explicit CCCD descriptor write for Android 13+
- OTA: start packet retry (previously no retry, blocked by pending CCCD)
- OTA: firmware path device-mode-aware (server/client subdirs)
- Removed stale root firmware.bin, flashMethodGroup, dialog_overwrite_confirm
- PrefsHelper: serverUUID/clientUUID persistence

android/app/build.gradle

@@ -8,9 +8,6 @@ def hasSigningConfig = signingPropsFile.exists()
 
 android {
     namespace 'nl.eddydevink.sylda'
-    // compileSdk/targetSdk stays on 34 (Android 14) by deliberate choice. The target audience
-    // consists of physically disabled users who are very reluctant to update their phones. Staying
-    // on API 34 maximises device compatibility. Re-evaluate when the user base has migrated.
     compileSdk 34
 
     if (hasSigningConfig) {
@@ -59,6 +56,31 @@ android {
     }
 }
 
+task generateSylDaVersions {
+    def versionFile = rootProject.file('../version.txt')
+    def outputFile = file('src/main/java/nl/eddydevink/sylda/SylDaVersions.kt')
+
+    inputs.file(versionFile)
+    outputs.file(outputFile)
+
+    doLast {
+        def versions = [:]
+        versionFile.eachLine { line ->
+            def parts = line.trim().split('=')
+            if (parts.length == 2) versions[parts[0].trim()] = parts[1].trim()
+        }
+        outputFile.text = """// Auto-generated from version.txt — DO NOT EDIT
+package nl.eddydevink.sylda
+
+object SylDaVersions {
+    const val SERVER = "${versions.server}"
+    const val CLIENT = "${versions.client}"
+}
+"""
+    }
+}
+preBuild.dependsOn generateSylDaVersions
+
 dependencies {
     implementation 'androidx.core:core-ktx:1.12.0'
     implementation 'androidx.appcompat:appcompat:1.6.1'