Show progress while duplicate candidates are hashed

[Muawiya-contact]

Goal

Show progress during duplicate hashing, not only the coarse phase label.

Large duplicate candidate sets can make the app feel busy for a while. Users should see that duplicate work is moving forward.

Current behavior

The app already maps report stages to labels:

ReportStage::FindingDuplicates => ScanPhase::CheckingDuplicates,

But it does not show how many duplicate candidates have been hashed.

Suggested design

Add a small progress contract around duplicate hashing, for example:

struct DuplicateProgress {
    candidates_seen: u64,
    candidates_total: u64,
}

This should stay approximate and should not fake a percentage unless the total is known.

Acceptance criteria

• Duplicate hashing emits useful progress for large candidate sets.
• The UI remains calm and does not jump layout.
• Cancellation still works during duplicate hashing.
• Tests cover the progress callback shape.