Rust: list_system_fonts and load_system_font Tauri commands using
font-kit crate for cross-platform system font access.
TS: font shim detects Tauri runtime and uses invoke() for font
listing/loading instead of queryLocalFonts (unavailable in WKWebView).
fflate's zipSync produces ZIP files that Figma rejects. Move the entire
fig-kiwi container + Zstd compression + ZIP packaging into a single
Rust command (build_fig_file). Zstd now includes content size via
set_pledged_src_size. Web fallback still uses fflate/deflate.