Rewrite ShareGo as one Flutter app

[MehdiMamas]

Summary

• Replace the React Native and Electron monorepo with one Flutter app that builds Android, iOS, macOS, Windows, and Linux downloads.
• Keep the two-device local session: the receiver shows a rotating code, the sender joins, the receiver approves, and short text is encrypted with XChaCha20-Poly1305.
• Publish those binaries from a v* tag. macOS and iOS builds stay unsigned until an Apple Developer account is added.

Test plan

• flutter analyze
• flutter test
• Install the release APK on an Android phone and pair it with another device on the same Wi-Fi
• Confirm a v* tag attaches apk, windows zip, macos dmg, unsigned ipa, and linux tarball to the GitHub Release
• User-facing text is in lib/strings.dart
• Timeouts and limits are in lib/config.dart
• No secrets are written to disk

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