Add setting to sync clipboard history across Macs via iCloud

[chamika]

Summary

Adds a Data → iCloud settings toggle that keeps clipboard history in sync on every Mac signed in to the same Apple Account, backed by a CloudKit private database.

• CloudSyncManager owns the preference, checks CKAccountStatus, and tracks whether the running store is actually CloudKit-backed.
• SwiftData cannot move a store between local and CloudKit at runtime, so the preference is read once while building the ModelContainer. Settings surfaces this and offers a Relaunch button rather than pretending the change is live.
• If the CloudKit store fails to open (missing entitlement, unusable account), the app falls back to the local store instead of trapping, and Settings explains why sync is off.
• DataSettingsView moved out of SettingsView.swift into its own file, matching the other settings views.

Sync is gated off in this build

Apple does not grant the iCloud capability to free personal development teams — Xcode refuses to issue a provisioning profile for it at all. So CloudSyncManager.isSupported is false, the entitlements file ships unreferenced, and the toggle is disabled with an explanation. The re-enabling steps (flip the flag, restore CODE_SIGN_ENTITLEMENTS + APS_ENVIRONMENT, add the capability in Xcode) are documented on the flag itself. project.pbxproj is unchanged by this PR.

Note on the schema change

#Unique<ClipboardItem>([\.contentHash]) is removed. CloudKit-backed stores reject unique constraints, and the model has to stay CloudKit-compatible regardless of the toggle since both modes share one schema — so this had to be unconditional. ClipboardMonitor now collapses duplicate hashes on lookup (keeping the newest, preserving a pin if any duplicate had one), which is equivalent for local-only use and additionally covers the same content being copied on two Macs.

Removing a uniqueness constraint is a lightweight Core Data migration, so existing local histories should open fine — but that is the one thing not verified against a real populated store.

Test plan

• Signed Debug build succeeds, no warnings
• Signed Release build succeeds, no warnings
• VoltPasteTests pass
• Signed binary entitlements verified: app-sandbox and files.user-selected.read-only intact, no iCloud keys, no stray .entitlements copied into the bundle
• End-to-end sync between two Macs — not possible until signed by a paid team

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