ccanvas 0.5.1

Release v0.5.1: fix macOS freeze when opening file/folder dialogs

The pick_dir/pick_file/pick_path commands were synchronous, so they ran
on the main thread and blocked it with blocking_pick_* — deadlocking on
macOS where the native panel itself needs the main thread. Making them
async moves the wait onto the runtime's thread pool, so opening a new
tab (folder picker) and file selects no longer hang the app.