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