Function generate_html 

pub fn generate_html(
    tree_json: &str,
    total_size: u64,
    file_count: usize,
    scan_path: &str,
    leaf_depth: u16,
) -> String

Generate an interactive HTML treemap using ECharts.

The generated HTML is self-contained (loads ECharts from CDN) with:

• Dark theme styling
• Breadcrumb navigation for drill-down
• leafDepth: 3 — shows 3 levels at a time
• Tooltips with full path and formatted size
• Hierarchical borders (thicker at top levels)
• Responsive resize