Phase 73.4 — Schema Designer UI

Two-row toolbar avoids clipping and narrow, vertical labels.
Explicit Close control in the header.
Resizable schema designer sheet with a practical minimum size.
Show/hide object sidebar; include/exclude individual tables and views.
All/None selection and search.
Four colour palettes (Ocean, Slate, Forest, Plum) applied to on-screen cards and PDF/PNG/SVG output.
Diagram-specific saved object selections and colour palette.
Optional full column display and auto-layout controls.
Existing printing and export retained.

The schema designer remains read-only and does not alter MySQL objects.

Full Xcode compilation and macOS UI verification are required.