Improved file provider switch user experience

[i2h3]

Makes the macOS File Provider and classic sync folders (FinderSync) mutually exclusive at the app level, since a single File Provider domain deactivates the FinderSync extension system-wide.

Key changes

• Single app-level Enable File Provider switch in General settings; the per-account File Provider panel is removed.
• File Provider mode on → every account (existing and later-added) gets a domain; off → all domains removed. Fresh installs default to File Provider; existing classic-only deployments stay classic.
• Enabling discards all classic sync-folder connections (local files kept on disk); disabling removes domains and does not recreate classic folders. Both directions require explicit confirmation.
• Startup reconciliation dialog for mixed deployments: Keep File Provider / Keep classic sync folders / Decide later (re-prompts until resolved).
• The account page's "Classic sync" panel is hidden while File Provider mode is on (with a conflict banner + Resolve… while classic folders still exist), and its E2E section shows "The File Provider extension does not support end-to-end encryption yet."
• Account wizard follows the global mode (skips folder setup when on) and cancels itself if the mode is toggled while it is open.
• Hardening: domain-add failures are now propagated (no more silent success), and FolderMan::addFolder refuses classic folders while the mode is on.

Manual test cases

1. Fresh install: enable → no folders, no confirmation folder-list; add account via wizard → sync-options step skipped, domain created; second account → domain created.
2. Enable with classic folders: confirmation lists affected accounts → Cancel leaves everything, switch snaps back; Enable → domains created, classic connections removed, local files kept, FinderSync badges gone, "Classic sync" panels hidden.
3. Disable: confirmation → domains removed, nothing recreated, "Classic sync" panel + Add Folder Sync Connection return. Dirty-data variant: unsynced edit first → "Unsynchronized Content" + preserved folder revealed.
4. Switch UX: switch disabled during the operation; rapid double-click harmless; cancelling a confirmation reverts the switch.
5. Mixed-deployment update: reconciliation dialog appears → verify all three buttons; Decide later shows the account banner and re-prompts next launch; kill app right after Keep File Provider → re-prompts (crash recovery).
6. Migration: existing per-account-FP deployment → stays on File Provider, no dialog; classic-only deployment → stays classic.
7. New account while mode on: wizard skips sync options and provisions a domain; toggling the switch while the wizard is open cancels the wizard.
8. E2E section: with File Provider on, every account page shows the "not supported yet" note (including servers without the e2e app); with File Provider off, the normal "set up encryption" prompt appears only where the server supports e2e.
9. Failure path: force a domain-add failure → classic folders left intact, warning lists failed accounts, retried/re-prompted next launch.
10. Platform gating: macOS 13 Ventura → switch hidden, classic available; Windows/Linux and non-File-Provider builds → unchanged.

Checklist

• Sign-off message is added to all commits.
• The commit history is clean with no merge commits.
  • How to rebase your commits
  • How to squash commits with rebase
• Uploaded screenshots from before and after for UI changes.
• Documentation has been updated or is not required.
• Backports requested where applicable (critical bugfixes).
• Labels added where applicable (bug/enhancement).
• Milestone added for target version.

AI (if applicable)

• The content of this PR was partly or fully generated using AI
  • I have read the guidelines for AI-assisted contributions.
  • I have read Nextcloud's AI Contribution Policy.

Screenshots

Description	Before	After
General Settings
File Provider Account
Sync Folder Account
Server lacks E2EE Capability	See sync folder account (section is omitted)

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
75.9% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)
D Security Rating on New Code (required ≥ A)
300 New Code Smells (required ≤ 0)
1 New Bugs (required ≤ 0)
D Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10354.zip

Digest: sha256:51abd7504466b07f55275685415916451f9866b36b69d5a5c524e22181cf9627

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.

[i2h3]

/backport to stable-34.0