fix: resolve auto-save race conditions and add conflict detection

[collinsezedike]

Summary

AutoSaveManager.tsx

• Replaced onSaveSuccess/onSaveError in the useEffect dep array with callback refs — inline functions from the parent were causing the manager to be torn down and rebuilt on every render, creating a race between the outgoing destroy and the incoming enable
• Added isSavingRef guard to the debounced save effect so concurrent saves (interval + debounce firing together) can't overlap

auto-save-manager.ts

• Named the online/offline handlers and stored them in instance fields so destroy() can properly removeEventListener — previously they leaked on every unmount
• Added conflict detection in saveNow: before writing, compares the stored draft's updatedAt against lastKnownUpdatedAt (set on load and after each successful save); aborts with an error status if the draft was modified externally (e.g. another tab)
• destroy() now also clears lastKnownUpdatedAt

Test plan

• Pass inline onSaveSuccess/onSaveError callbacks from a parent — manager should not rebuild on every render
• Trigger rapid form changes — only one save should be in flight at a time
• Open the same form in two tabs, edit in one, then edit in the other — second save should surface a conflict error
• Unmount the component — no online/offline listeners should remain attached

Closes #195

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