Guard CloudKit startup in non-entitled builds

[tymrtn]

Problem

Flycut can crash when iCloud sync is enabled in builds that do not have valid iCloud entitlements (common in local/unsigned debug workflows).

Root cause

MJCloudKitUserDefaultsSync assumes CloudKit startup is always valid, then proceeds into container/account setup paths that can throw/trap when:

• container identifier is missing/invalid, or
• process lacks iCloud container entitlement.

Solution

Harden CloudKit startup in MJCloudKitUserDefaultsSync by:

1. Resolving a safe effective container identifier (fallback to iCloud.<bundleIdentifier> when needed).
2. Checking process entitlement (com.apple.developer.icloud-container-identifiers) before enabling CloudKit.
3. Guarding container creation/account-status calls with nil/exception handling.
4. Exiting early and cleanly (resume queue + decrement user action) when prerequisites are not met.

Files changed

• MJCloudKitUserDefaultsSync/MJCloudKitUserDefaultsSync/MJCloudKitUserDefaultsSync.m

Why this is safe

• No behavior change for correctly signed/entitled builds.
• Only affects failure paths by preventing crash and skipping invalid CloudKit enablement.

Validation

• Reproduced original crash scenario in unsigned workflow before fix.
• After fix, CloudKit startup exits safely instead of crashing.
• Framework target builds successfully:
  • xcodebuild -project MJCloudKitUserDefaultsSync/MJCloudKitUserDefaultsSync.xcodeproj -scheme MJCloudKitUserDefaultsSync -configuration Debug build

Additional context

Top-level app build on Xcode 26 may hit an unrelated existing project build-cycle warning in this repository; this PR intentionally does not change project build-phase wiring and is scoped only to the CloudKit crash guard.

Closes #321