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/app.json

@@ -47,16 +47,12 @@
           "color": "#6366F1",
           "sounds": []
         }
-      ]
+      ],
+      "./plugins/withAndroidReleaseSigning"
     ],
     "experiments": {
       "typedRoutes": true,
       "reactCompiler": true
-    },
-    "extra": {
-      "eas": {
-        "projectId": "aeaaa2bd-3a27-4771-8e39-f2e14fe0e030"
-      }
     }
   }
 }