build: remove expo.dev/EAS dependency, add local Android build pipeline

- Delete eas.json and strip extra.eas.projectId from app.json
- Add plugins/withAndroidReleaseSigning.js — Expo config plugin that injects
  release signingConfig into the generated build.gradle during expo prebuild
- Add build-apk.sh — self-contained build script (expo prebuild + Gradle)
  Reads keystore credentials from ~/.config/postiz-mobile/signing.env
  Outputs APK/AAB to dist/, wipes credentials from gradle.properties after build
- Add install-android-sdk.sh — one-time Android SDK cmdline-tools bootstrap
- Remove unused expo-task-manager dependency
- Update .gitignore: android/, ios/, static-build/ excluded (generated)

Build workflow:
  1. eas credentials --platform android  # export keystore once
  2. ./install-android-sdk.sh            # first time only
  3. ./build-apk.sh                      # → dist/postiz-mobile-YYYYMMDD-HHMM.apk

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

artifacts/postiz-mobile/package.json

@@ -61,7 +61,6 @@
     "expo-clipboard": "~8.0.8",
     "expo-notifications": "~0.32.17",
     "expo-secure-store": "~15.0.8",
-    "expo-task-manager": "~14.0.9",
     "react-native-calendars": "^1.1314.0"
   }
 }