Replace Cloudflare KV sync with browser storage.sync

[patrickiel]

Cloudflare KV is too expensive for a free extension. Move cross-device sync to the browser's own storage.sync and drop the server.

• No server, no sync ID, no cookies permission; delete server/ and its CI job.
• Snapshot is gzipped, base64-chunked into ≤ 8 KB items, and trimmed by priority to the 100 KB quota (charts first, then unlisted tracks, then Recent). Nothing is deleted locally by trimming; applying merges per track.
• Chord charts travel in a compact form (centisecond delta arrays).
• Settings shows usage (% of 100 KB) and warns on Chrome when the profile isn't signed in with sync on (identity permission, no install warning).
• Empty per-track records are pruned.
• Migration removes the old syncId and re-seeds; old Worker/KV to be deleted by hand.