Rebuild file transfer as a self-contained core with resume

[ctoth]

Summary

Replaces FileTransferManager, WebRTCService and FileTransferStore with a new core in src/fileTransfer/. The engine knows nothing of GMCP, WebRTC or IndexedDB; those sit behind a signalling, a link and a storage interface, and createFileTransfers.ts assembles them for the browser.

What changes for users

• Several transfers at once. Each transfer has its own connection, and the panel shows a row with its own progress and Cancel button for each.
• Resume. After a dropped link or a page reload, only the missing pieces are sent, and the receiver is not prompted again.
• Truthful completion. The sender reports success once the receiver has verified and saved the file.
• The pending-offer row shows the file size.
• Offer notifications name the sender by player name, and are spoken when desktop notifications are not granted.

How it works

• Content id: SHA-256 over the SHA-256 digests of consecutive 16 KiB pieces, so a file is hashed and verified one piece at a time.
• Bounded memory: received pieces go straight to storage, and the sender keeps at most 64 pieces (1 MiB) outstanding until the receiver reports them stored.
• One connection per transfer: ICE candidates carry the transfer id inside the GMCP candidate string so they can be routed.
• Resume: the sender re-offers the same id marked as a resume; the receiver answers without prompting and says which pieces it holds.
• Closing: the side that sends the last frame keeps the link open until the peer closes it.

Trust

• No connection exists on the receiving side before the user accepts.
• Name, size and content are checked against the accepted offer; the name shown and saved comes from the offer, with path components, control characters and bidi/format characters removed.
• Undecided offers are capped per peer and expire; re-offers and confirmations are bounded; held ICE candidates are capped.
• History keeps the last 50 finished transfers in the order they finished; an offer nobody accepted is not kept.

Client and UI

• client.getFileTransfers() replaces getFileTransferManager(); client.onFileTransferOffer() reports each new incoming offer, and is what notifications and the sidebar now use.
• RequestResend is removed from the Client.FileTransfer package.
• The panel renders from engine snapshots (useTransfers, useTransferHistory).

Accessibility

• History lines are spoken through the client's announcer, so they are heard whichever sidebar tab is showing.
• Focus moves to the panel heading when the row that held it disappears.
• The recipient combobox always has its listbox, marks the highlighted option as selected, and announces when nobody matches.
• Rows are named groups; Cancel buttons say what they cancel; the progress bar says when it is connecting or verifying.
• The file input is labelled; the open panel no longer clips at a fixed height.

Compatibility

The data-channel format, candidate tagging and content id are new, so this build and older builds cannot exchange files with each other.

Docs

docs/GMCP/packages/client-file-transfer.md describes the content id, the tagged candidate, the resume marker and the data-channel protocol.

Verification

• npm run typecheck: clean.
• npx vitest run: 135 files, 2,061 tests pass.
• npm run build: the transfer code is its own lazily loaded chunk, createFileTransfers-*.js, 28.7 kB.