ui: add "Add New (copy settings)" button and initialdir support for file browser

[Copilot]

When adding multiple scans that share most settings (orientation, staining, session, etc.), users had to re-enter everything for each new scan. The file browser also always opened to the OS default directory instead of the last-used path.

Changes

• _browse_for_file(initialdir=None) — passes initialdir to tkinter.filedialog.askopenfilename; updated on each successful selection so subsequent Browse clicks open the same folder
• _render_scan_entry(..., *, copy_settings=None) — pre-populates session-state widget keys for orientation, sample, session, acquisition, and stain channels from copy_settings on first render; also seeds initialdir from the source path's parent directory
• "➕ Add New (copy settings)" button — captures all fields from the last scan except spim_path and subject, stores them as _copy_settings_{new_id} in session state, then appends the new scan and reruns; disabled when no scans exist
• non_empty_rows filtering — strips _-prefixed internal keys (e.g. _staining_channels_raw) before passing rows to validators/generators