Show printer name in spool modal title for multi-printer setups

The modal title now reads "Printer Alpha · Box 1 · Slot A" instead of
just "Box 1 · Slot A", making it clear which printer a slot belongs to.
Falls back to slot label only when no printer name is available.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>