fix(sync): keep status responsive during indexing

[veryCrunchy]

Closes #234

Outcome

• Keep Sync & offline usable while local media discovery runs.
• Return the latest durable Transfer Center snapshot without waiting for an active upload or indexing pass.
• Reconcile interrupted transfers only when the sync engine is idle.
• Prevent Refresh from starting overlapping MediaStore scans.
• Avoid rescheduling a removed sync pair from a stale presentation snapshot.

Validation

• Focused Android engine and atomic-store tests pass.
• Focused desktop FileOfflineCenter tests pass.
• Repository hygiene and changelog validation pass.
• An independent concurrency review was completed and its scheduling-race and scan-coalescing findings were fixed before opening this PR.

[chatgpt-codex-connector]

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