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/eas.json

@@ -0,0 +1,11 @@
+{
+  "cli": {
+    "version": ">= 16.0.0"
+  },
+  "build": {
+    "preview": {
+      "android": { "buildType": "apk" },
+      "ios": { "simulator": true }
+    }
+  }
+}