Summary

Exporting a backup from Settings felt like a freeze on a large database. The export CPU/IO already ran off the main thread (SwiftDataStore is a @ModelActor; the zip is a Task.detached(priority: .utility)), so the freeze was really the system share sheet's modal "Preparing…" phase: the old ShareLink used a lazily-built Transferable, so tapping Export presented the share sheet immediately and left it blocking in "Preparing…" while the archive built.

This builds the archive up-front on a background task, streaming progress to an in-app "Exporting…" bar (mirroring import), then reveals a ShareLink to the ready file so the share sheet opens instantly.

Changes

• BackupCoordinator.exportBackup gains an onProgress callback: the evidence-blob load drives the determinate leg (0…0.8, whole-percent throttled like import), then jumps to 1 once the opaque encode + zip writes the archive.
• BackupModel.exportBackup marshals that progress onto backupProgress via the same AsyncStream + @MainActor observer pattern as importBackup.
• SettingsView replaces the lazy ShareLink with an Export button (showing the progress label while exporting) plus a revealed "Share backup" ShareLink over the ready file. importProgressLabel is generalized to backupProgressLabel(_:systemImage:), reused by both rows.
• BackupArchiveFile is now a non-lazy Transferable wrapping the ready URL, so the .zip content type stays explicitly declared rather than inferred from the filename.
• 10-minute expiry: a .task(id: exportedArchiveURL) hides the "Share backup" row and deletes the temp file after a retention window, so a stale link can't be shared. Deletion routes through the new BackupCoordinator.discardExport() (which owns the temp-file lifecycle); BackupModel.discardExport() forwards to it.
• Adds settings.backup.exporting ("Exporting…") and settings.backup.share ("Share backup") strings.

Notes / trade-offs

• Progress is only truly determinate over the evidence-blob load; the JSON-encode + zip is a single atomic step, so the bar holds near 0.8 then completes — inherent to one opaque zip call, documented in code.
• Stays ShareLink-based (one extra tap: Export → then Share) rather than a UIActivityViewController bridge. As a consequence, "hide when the share sheet closes" isn't wired — ShareLink exposes no dismissal callback — so the temp file is reclaimed by the 10-minute expiry (or the next export) instead.

Testing

• BackupCoordinatorTests.exportReportsProgressUpToCompletion — progress is non-empty, in (0, 1], and ends at 1.
• BackupCoordinatorTests.discardExportDeletesTheExportDirectory — the temp directory is gone after discardExport, and a second call is an idempotent no-op.
• Existing BackupModelTests / backup round-trip tests still pass.
• ./swiftformat --lint clean; WhereCoreTests and WhereUITests pass locally.

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