docs(flutter): rewrite client/flutter/README.md

[pentaxis93]

Summary

Replace the 17-line Flutter boilerplate README in client/flutter/ with real content that describes what the app does, its features, supported platforms, and how to build it. A developer exploring the repo will navigate into this directory — the boilerplate is an immediate credibility hit.

Part of: #3

Scope

• client/flutter/README.md — replace entirely with: what the app does, supported platforms (Android, Web, Linux, Windows), key features (offline queue, home widget, amplitude visualization, secure key storage), build instructions per platform, link to BUILD.md for full details

Acceptance criteria

• client/flutter/README.md contains project-specific content (no Flutter boilerplate)
• Covers: app description, supported platforms, key features, build quickstart
• References BUILD.md for detailed build instructions
• Accurate — platform support and features match actual codebase capabilities
• dart analyze in client/flutter/ still passes

Dependencies

None — Layer 0, no blockers.