Upgrade Flutter dependencies and migrate affected client integrations #138

Upgrade the client to the newest mutually resolvable Flutter package versions on the pinned Flutter 3.47.9 SDK. Migrate file selection/saving and chart APIs, update PowerSync test fixtures and browser assets, and refresh native plugin registrations and lockfiles.

Acceptance criteria:

• Dependency resolution is reproducible with the committed lockfile; app version and reader Git pin remain unchanged.
• Import, attachment, photo selection and save flows support the current file-picker API, including bounded streams and Android content URIs.
• Formatting, analysis, client tests, browser checks, and Android/web builds pass.
• Document packages blocked by upstream constraints and platform checks not performed.

Maintenance task; no new product requirement or server contract change.