fix: Files don't update when external downloads complete (Chrome, etc.)

[BrianLeishman]

Summary

When downloading files via Chrome (or other apps), the file doesn't update in Marlin when the download completes. The temporary/partial download file remains shown instead of the final file.

Current Behavior

• Chrome downloads show as .crdownload files (or similar temp files)
• When download completes, Chrome renames the file to its final name
• Marlin doesn't reflect this change - still shows the old temp file
• May need to manually refresh to see the real file

Expected Behavior

• File watcher should detect the rename/replacement
• UI should update automatically to show the final file
• Thumbnail should generate for the new file type

Likely Causes

1. File watcher not catching renames: May only watch for create/delete, not rename events
2. Debouncing too aggressive: Change events may be getting dropped
3. Temp file delete + new file create: If Chrome deletes temp and creates new (vs rename), both events need handling
4. Platform-specific: Different behavior on macOS/Windows/Linux file watchers

Files to Investigate

• File watcher implementation in Rust backend
• How rename events are handled vs create/delete
• Debouncing/batching logic for file system events

Test Cases

• Chrome .crdownload → final file
• Firefox .part → final file
• Safari download completion
• Any app that writes to temp then renames

Acceptance Criteria

• Chrome downloads update automatically when complete
• Other browser downloads work similarly
• No manual refresh needed
• Thumbnail generates for the final file
• Works on macOS, Windows, and Linux