Move settings into a cogwheel modal; remove inline settings card

Replaces the always-visible "Spoolman sync mode" card with a ⚙ button
in the topbar that opens a settings modal. The modal is structured with
named sections (Spoolman) so new settings can be added there over time.
Spoolman section is hidden when Spoolman is not configured.

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