docs/feat: improve manual MainApplication setup docs and explore autolinking hooks

[imsankalp]

Problem

Non-Expo users on bare React Native must manually edit MainApplication.kt / MainApplication.java to add the OkHttp client builder — a native code change that is error-prone, easy to forget, and not validated at install time. There is no autolinking hook that handles this automatically for bare RN.

Current setup docs are brief and lack:

• A clear before/after diff of MainApplication
• An explanation of why the call must go in onCreate() and not elsewhere
• The required BuildConfig.DEBUG guard (which is also a separate bug — see bug(android): NETWORK_TOOLS_ENABLED is always true — interceptor runs in production builds #14)
• Troubleshooting steps for common mistakes (wrong placement, missing imports, Java vs Kotlin differences)

Proposed improvements

1. Expand SETUP_GUIDE.md with a step-by-step native setup section

Include:

• Full before/after diff for both Kotlin and Java MainApplication
• Explanation of NetworkingModule.setCustomClientBuilder and why it must be called before loadReactNative
• The BuildConfig.DEBUG guard with rationale
• Common errors and how to fix them (e.g. ClassNotFoundException, interceptor not capturing requests)

2. Explore React Native autolinking hooks

React Native's autolinking system supports postInstall hooks in react-native.config.js or package-level scripts. Investigate whether we can auto-patch MainApplication for bare RN apps on npm install / pod install — similar to what the Expo config plugin does for Expo.

Options to evaluate:

• @react-native-community/cli hooks
• A postinstall script that detects bare RN and applies the same regex patch as plugin/src/index.js
• A dedicated npx react-native-network-tools setup CLI command

3. Add setup validation

A helper that warns at runtime if the interceptor was not registered:

// In NetworkMonitorProvider's useEffect
if (isNativeNetworkToolsAvailable() && getRequestCount() === 0 && __DEV__) {
  console.warn(
    '[NetworkTools] No requests captured yet. Did you call ' +
    'NetworkingModule.setCustomClientBuilder() in MainApplication?'
  );
}

Acceptance criteria

• SETUP_GUIDE.md has a full before/after diff for Kotlin and Java MainApplication
• BuildConfig.DEBUG guard documented and explained in setup guide
• Troubleshooting section added covering the 3 most common setup mistakes
• Autolinking hook options evaluated — decision documented in ARCHITECTURE.md
• Runtime warning implemented when interceptor appears unregistered in dev mode
• README quick-start section reflects the updated setup steps