Harden saved thread storage and navigation stability

[infinityf4p]

Summary

• move large saved-thread mutations and media resolution off the main actor while serializing sync and async commits through one revisioned coordinator
• recover interrupted media transactions without deleting the only rollback copy, validate snapshots and backup packages more strictly, and clean text-only media safely
• remove unreachable navigation gesture activation code, fix standalone search dismissal ordering, and reconcile imported font catalogs/files

Verification

• all non-live TiebaPureTests passed on iOS 18.3
• saved-thread regression suite and final save/open UI flow passed
• four save/search/navigation UI flows passed on iOS 16.4
• Xcode static analysis passed
• unsigned Release build passed; app and extension minimum OS are 16.4 and SwiftData remains weak-linked

Live-account posting/reply tests were not run because these changes do not touch authenticated network writes.