fix: add AppleButton.js so Expo Metro can resolve the export (#375)

[cpruijsen]

Fixes #375
Fixes #364

Why

lib/index.js always does export { default as AppleButton } from './AppleButton', but the published package only shipped AppleButton.ios.js / .android.js / .macos.js (and AppleButton.shared.js constants). There was no generic AppleButton.js. Expo SDK 52 Metro (EAS, including the expo export:embed server pass) resolves that specifier with web/generic source extensions and never tries .ios.js, so the app fails to bundle even when it only uses appleAuth.

Reporter listed the missing file and the exact Metro candidate list on #375. The same error is #364 (iOS Bundled succeeded, then the server/web pass failed). A commenter confirmed a Platform.select + require() workaround against node_modules.

How

Add lib/AppleButton.js as the platform-agnostic module those sourceExts look for. It Platform.selects the existing iOS / Android / macOS implementations, with the Android JS button as default.

index.js is unchanged. When Metro still applies platform suffixes, native iOS/Android/macOS keep using the existing platform files.

Decision

• Chose: new AppleButton.js fallback with Platform.select.
• Alternative: put Platform.select in index.js (the patch on AppleButton module resolution fails in EAS Build due to missing AppleButton.js file #375), or re-export AppleButton.android.js with no select (same shape as AppleButton.macos.js).
• Why: a generic file is the React Native fallback Metro expects; Platform.select keeps the native iOS button if Expo loads the generic file on iOS. default is the Android JS button, not AppleButton.shared.js, that file is constants only and has no default export, so the snippet in the issue would have set AppleButton to undefined.
• Happy to switch to either alternative if that is preferred.

Test plan

• Resolution test using the Expo web sourceExts from AppleButton module resolution fails in EAS Build due to missing AppleButton.js file #375 fails without AppleButton.js and passes with it (yarn test)
• yarn lint / yarn tsc:compile
• npm pack --dry-run includes lib/AppleButton.js and omits __tests__/
• EAS / Expo SDK 52 expo export:embed (reporter's environment) still succeeds and iOS still shows the native button

[CLAassistant]

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