refactor(hub): drop the base64 chunk fallback and the dead HTTP file client

With MNP 0.15 no node can emit a base64 `file_chunk`, so the browser's fallback
for that shape is unreachable. Three things go with it:

- `file-utils.js` kept a third branch below the fallback that base64-decoded
  `chunkMsg.ct_b64 || chunkMsg.data_b64` when neither was present, i.e. decoded
  `undefined` and wrote the result into the file the user was saving. A chunk we
  cannot decrypt now stops the download with an error naming the file and
  suggesting the node is older than the page. Deliberately not in
  `_isRetryableTransportError`: this is a version mismatch, not a bad moment on
  the link.
- `crypto.js` `decryptChunk` (base64) was the real path until Phase 9.15 and has
  had no caller since.
- `crypto.js` `decryptFile` was never called in any commit. It fetched
  `${nodeUrl}/file/${id}/${chunk}?token=` in a loop — the node's unauthenticated
  HTTP file API, which is finding C1 and was deleted in Phase 11.5. A client for
  an endpoint that no longer exists, kept alive by being exported.

`decryptChunkBin` — every file download and every video segment — is untouched.

`packages/meshbay-client/ui/` was resynchronised with `npm run sync-ui`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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