Add recovery path for invalid local iCloud share state

[barronlroth]

Parent: #82
Related: #81

Problem

If a failed share attempt leaves invalid local CloudKit/Core Data metadata, Cubby currently lacks an explicit recovery path. Reinstalling clears local persisted state, which can make the app usable again, but it does not prove CloudKit contains clean records or that the app can self-recover.

Proposed work

• Add startup or on-demand diagnostics for inconsistent share state.
• Detect incomplete shares, missing share metadata, duplicate local records, and private/shared-store mismatches.
• Decide which states can be repaired automatically without deleting user data.
• Add a safe user-facing recovery path for unrecoverable local share state.
• Document a manual QA script for reproducing and collecting logs from old-home sharing failures.

Acceptance criteria

• A failed or partial share attempt cannot permanently crash-loop the app.
• Recoverable local state is repaired or quarantined without data loss.
• Unrecoverable state shows clear recovery copy and logs enough detail for support/debugging.