fix share count changes

[bretth18]

Description

This PR introduces a robust, multi-subscriber notification system for share count changes, ensuring that share statistics are reliably broadcast to the server and UI after any folder or file index update. It also improves resource management, avoids retain cycles in async tasks, and makes share state persistence safer and more testable.

Share count change notifications and broadcasting:

• Added a debounced, multi-subscriber countsChangesStream() to ShareManager using fanned-out AsyncStream continuations, with atomic notification after folder add/remove/scan and a 200ms debounce to coalesce rapid changes. [1] [2] [3] [4]
• NetworkClient now consumes countsChangesStream() in a dedicated task, automatically re-broadcasting share counts to the server whenever the local share index changes—even while disconnected—eliminating race conditions where the server would report zero shared files after login. [1] [2] [3]

Persistence and resource management improvements:

• ShareManager now accepts an injectable UserDefaults store, improving testability and preventing test cross-talk, and all persistence/load methods now use this injected store. The loadPersistedFolders() method is split from scanning so notification subscribers can be registered before a rescan. [1] [2] [3]
• Ensured security-scoped resource access is correctly reference-counted, avoiding leaks on redundant folder adds.

Async task and connection handling:

• All async consumer tasks in NetworkClient now use weak self references per iteration to avoid retain cycles, ensuring tasks terminate cleanly when the client is deallocated. [1] [2]
• In UploadManager, improved error handling for short reads and best-effort EOF signaling, surfacing partial transfers as failures and swallowing expected half-close errors when the peer disconnects first. [1] [2]

App initialization changes:

• App startup now wires up share count change consumers before loading persisted folders and rescanning, guaranteeing that initial share statistics are broadcast and observed.

How to test

• Ensure all checks pass

Author checklist

This PR:

• Satisfies a goal that is specific & clearly motivated
• Adds value in isolation (whether user-facing or sustainability-related)
• Contains a concise & easy-to-understand title + description
• Adheres to SRP by default
• Presents the best possible implementation to meet its goal, given constraints at hand