Add error logging for file tree I/O failures

[Copilot]

Addresses feedback on PR #6 to log I/O errors during file tree construction. Previously, errors when reading directory entries or metadata were silently ignored, making it difficult to troubleshoot missing files.

Changes

• Added error logging for subdirectory read failures using eprintln!
• Changed from if let Ok() to match statement to handle both success and error cases
• Errors when reading individual directory entries or metadata still use if let Ok() pattern (non-critical - these entries are simply skipped)

Example

// Before
if let Ok(sub_children) = build_tree_recursive_desktop(&path) {
    children = sub_children;
}

// After  
match build_tree_recursive_desktop(&path) {
    Ok(sub_children) => {
        children = sub_children;
    }
    Err(e) => {
        eprintln!("Failed to read subdirectory '{}': {}", path, e);
    }
}

This provides visibility into permission issues or I/O errors for subdirectories while maintaining the existing behavior of silently skipping individual problematic entries.

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