quantmodel — Add optional authenticated sync with explicit local merge

Objective

Add optional authenticated sync with explicit local merge.

Notes: Added feature-gated Supabase password sign-in/PostgREST sync on the discoverable Custom Meta page. First sync is a read-only explicit preview; remote-only rows merge locally, local-only rows upload, name conflicts are reported and local wins only after confirmation. Local storage/JSON backup remain default. Node and Chromium tests pass; live two-user proof stays t290. See research/findings/account-sync-t289.md.

Approach

Reuses existing report research/findings/account-sync-t289.md — tables/metrics preserved verbatim via source_report pointer. Body below summarizes grounded evidence from that report and adjacent artifacts.

Files / code / data changed

No commit SHA confidently associated with t289 in sampled log [reconstructed — uncertain].
Source report: research/findings/account-sync-t289.md
Task-history entry: research/findings/task-history.json#t289

Results

Conclusions and metrics are in research/findings/account-sync-t289.md; see detail page for preserved tables/metrics.

Conclusions

[reconstructed — uncertain] Outcome inferred from status done and task note.

Problems / follow-ups

See source report or PR discussion for follow-ups. No unsupported follow-ups fabricated.