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/eas.json

@@ -1,24 +0,0 @@
-{
-  "cli": {
-    "version": ">= 16.0.0"
-  },
-  "build": {
-    "preview": {
-      "distribution": "internal",
-      "android": {
-        "buildType": "apk"
-      }
-    },
-    "production": {
-      "android": {
-        "buildType": "app-bundle"
-      },
-      "ios": {
-        "resourceClass": "m-medium"
-      }
-    }
-  },
-  "submit": {
-    "production": {}
-  }
-}