docs: rewrite README for local build workflow, drop EAS references

- Remove EAS/expo.dev from prerequisites and build sections
- Document build-apk.sh workflow step by step
- Document first-time setup (Android SDK + keystore export + signing.env)
- Update architecture tree (add lib/extractError.ts, remove expo-task-manager)
- Remove Replit-specific sections (dev server, push via bundle, env vars)
- Keep minimal eas.json (needed only for one-time keystore export via eas credentials)

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

artifacts/postiz-mobile/app.json

@@ -47,11 +47,16 @@
           "color": "#6366F1",
           "sounds": []
         }
-      ],
+      ]
     ],
     "experiments": {
       "typedRoutes": true,
       "reactCompiler": true
+    },
+    "extra": {
+      "eas": {
+        "projectId": "aeaaa2bd-3a27-4771-8e39-f2e14fe0e030"
+      }
     }
   }
 }