feat(convert): default scaled zarr to <series>.zarr beside the source images

[dustenhubbard]

Improves the default when converting images to a scaled zarr (the save dialog still lets the user pick any location/name — this only changes the suggested default).

• Location: default is now a sibling of the source image directory (one level up), so the .zarr sits next to the images rather than inside them.
• Name: <series>.zarr (was the hard-coded <series>_images.zarr).
• Filter loosened to *zarr so an overridden name like proj-zarr isn't turned into proj-zarr.zarr by the dialog's suffixing.
• Invariant enforced in code: ensure_zarr_suffix guarantees the chosen path ends in zarr (what ImageLayer.is_zarr_file = src_dir.endswith("zarr") requires), independent of the dialog's platform-dependent suffix behavior.

Pure path logic lives in a Qt-free helper (backend/func/zarr_naming.py) with unit tests; full suite green.