Require folder destination for receiver downloads over 7GB

[derekShaheen]

Motivation

• Large transfers and very large files can exhaust browser memory or fail when held in-memory during WebRTC -> blob download, so streaming directly to disk is needed for reliability.
• The UX should require a folder target when the total selected download exceeds a safe threshold to avoid high likelihood of failure for transfers >7GB.
• Browsers without the File System Access API must continue to work with the existing in-memory + Save-link fallback.

Description

• Added a Choose folder button and contextual helper text in public/pages/receiver.html to surface folder-based download options to the receiver.
• Implemented folder-targeted streaming in public/assets/scripts/receiver.js using the File System Access API (showDirectoryPicker, getFileHandle, createWritable) and a streamWriter path to write incoming binary chunks directly to disk.
• Introduced thresholds LARGE_TRANSFER_THRESHOLD = 7 * 1024 * 1024 * 1024 and LARGE_FILE_THRESHOLD = 8 * 1024 * 1024 * 1024 and enforced that if selected total bytes > 7GB the receiver must choose a folder before starting, while recommending folder use when any single file > 8GB.
• Preserved the existing in-memory Blob + Save-link flow for unsupported browsers or smaller transfers and added UI updates via updateFolderUi() and selection handlers to reflect folder state.

Testing

• Ran node --check public/assets/scripts/receiver.js and it completed successfully.
• Ran node --check server.js and it completed successfully.
• Started the app with npm start and verified the server listens on http://0.0.0.0:3000 (startup succeeded).
• Captured a Playwright screenshot of the receiver page at http://127.0.0.1:3000/receiver.html to validate UI render (screenshot saved as an artifact).

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Codex Task