fix: force JAVA_HOME to ~/jdk21 when system Java ≥ 25

Gradle 8.x supports up to Java 24. Fedora 44 ships Java 25 by default,
causing "Unsupported class file major version 69" at build time.

build-apk.sh now auto-detects system Java version and falls back to
~/jdk21 (Temurin 21 LTS) when Java ≥ 25 is detected.
README documents the one-time JDK 21 install step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/postiz-mobile/README.md

@@ -26,7 +26,7 @@ Build is fully local — no expo.dev account or EAS cloud required.
 |------|---------|
 | Node.js | 20 LTS |
 | pnpm | 10+ |
-| Java (JDK) | 17+ |
+| Java (JDK) | 17–24 (Java 25+ not yet supported by Gradle 8) |
 | Android SDK | see below |
 
 No expo.dev account needed for builds.
@@ -57,7 +57,19 @@ Scan the QR code with Expo Go on Android to preview the app live.
 
 ### First-time setup
 
-**1. Android SDK**
+**1. Java 21 LTS**
+
+Gradle 8 requires Java ≤ 24. If the system Java is 25+ (Fedora 44), install Temurin 21 locally:
+
+```bash
+wget -O /tmp/jdk21.tar.gz \
+  "https://github.com/adoptium/temurin21-binaries/releases/download/jdk-21.0.7%2B6/OpenJDK21U-jdk_x64_linux_hotspot_21.0.7_6.tar.gz"
+mkdir -p ~/jdk21 && tar -xzf /tmp/jdk21.tar.gz -C ~/jdk21 --strip-components=1
+```
+
+`build-apk.sh` will use `~/jdk21` automatically if the system Java is ≥ 25.
+
+**2. Android SDK**
 
 ```bash
 cd artifacts/postiz-mobile

artifacts/postiz-mobile/build-apk.sh

@@ -48,7 +48,20 @@ KEYSTORE_PATH_EXPANDED="${KEYSTORE_PATH_EXPANDED/#~/$HOME}"
 [ ! -f "$KEYSTORE_PATH_EXPANDED" ] && \
   abort "Keystore not found: $KEYSTORE_PATH_EXPANDED\n\nExport it from EAS:\n  eas credentials --platform android\n  → Download existing keystore → save to $KEYSTORE_PATH_EXPANDED"
 
-# ─── 2. Android SDK ────────────────────────────────────────────────────────
+# ─── 2. Java 17/21 — Gradle requires Java ≤ 24 (system Java 25 is too new) ──
+# Use ~/jdk21 if present, otherwise rely on JAVA_HOME already being set correctly.
+if [ -z "${JAVA_HOME:-}" ] || "$JAVA_HOME/bin/java" -version 2>&1 | grep -qE '"(2[5-9]|[3-9][0-9])\.'  ; then
+  if [ -d "$HOME/jdk21" ]; then
+    export JAVA_HOME="$HOME/jdk21"
+    export PATH="$JAVA_HOME/bin:$PATH"
+    info "Using local JDK 21: $JAVA_HOME"
+  else
+    abort "Java 25+ detected but Gradle only supports ≤ Java 24.\n\nInstall JDK 21 locally:\n  wget -O /tmp/jdk21.tar.gz https://github.com/adoptium/temurin21-binaries/releases/download/jdk-21.0.7%2B6/OpenJDK21U-jdk_x64_linux_hotspot_21.0.7_6.tar.gz\n  mkdir -p ~/jdk21 && tar -xzf /tmp/jdk21.tar.gz -C ~/jdk21 --strip-components=1"
+  fi
+fi
+info "Java: $("$JAVA_HOME/bin/java" -version 2>&1 | head -1)"
+
+# ─── 3. Android SDK ────────────────────────────────────────────────────────
 if [ -z "${ANDROID_HOME:-}" ]; then
   for candidate in "$HOME/android-sdk" "$HOME/Android/Sdk" "/opt/android-sdk"; do
     if [ -d "$candidate/platform-tools" ]; then
@@ -65,7 +78,7 @@ fi
 export PATH="$PATH:$ANDROID_HOME/platform-tools:$ANDROID_HOME/cmdline-tools/latest/bin"
 info "Android SDK: $ANDROID_HOME"
 
-# ─── 3. expo prebuild ──────────────────────────────────────────────────────
+# ─── 4. expo prebuild ──────────────────────────────────────────────────────
 info "Running expo prebuild (Android)…"
 cd "$SCRIPT_DIR"
 pnpm exec expo prebuild --platform android --clean --no-install