[SYNC-009] Conflict review center

[veryCrunchy]

Roadmap

• Task ID: SYNC-009
• Priority: P0

Problem

A conflict cannot be reviewed safely from filenames or raw metadata alone. Users need enough visual and contextual evidence to distinguish the local and remote versions, understand what changed, and choose a non-destructive resolution.

Acceptance criteria

• Conflict rows show bounded local and remote previews when available, with clear placeholders for unsupported, missing, permission-limited, cloud-only, RAW, and video content.
• The detail view presents local and remote size, modified time, content type, checksum or ETag evidence when available, account/destination context, and the operation that produced the conflict.
• Users can choose local, remote, keep both, propagate deletion, restore, or skip only when the operation is valid for that conflict type.
• Keep both produces a deterministic non-colliding name and preserves both originals.
• Replace and deletion choices use verified preconditions and never silently overwrite a version that changed during review.
• Preview decoding and fetching are bounded, cancellable, cached by version identity, and never block the paged conflict list.
• Transfer Center, Sync status, notifications, and file detail surfaces deep-link to the same authoritative conflict record.
• Resolutions survive process death, expose progress and failure recovery, and re-read authoritative state before marking the conflict complete.
• Tests cover identical-looking files, same-name different-content files, RAW/JPEG and Live Photo pairs, videos, cloud-only media, permission loss, stale ETags, deletion conflicts, and ambiguous network results.