refactor(settings)!: rebuild the theme engine on a slot registry - #2791

Why

Theme color groups shipped editable and read by nothing (Interface, Sidebar, Toolbar), and a malformed theme file loaded silently as Default Light. #2775 and #2781 fixed both, were taken back out by #2790 for review, and a later merge of main into this branch had dropped the engine again.

Change

• Re-lands refactor(settings)!: rebuild the theme engine on a slot registry with a schema gate #2775 and feat(settings): theme the content panes outside the editor and the data grid #2781 on current main: ThemeSlot registry, ThemeCatalog, ThemeResolver, schema 2 theme files with Not Loaded reasons, fonts moved to Settings > Editor and Data.
• Follows the macOS 13 port: ObservableObject instead of Observation, with every view that reads the palette observing the engine.
• Keeps main's newer work: VoiceOver names and log privacy in the theme panes, the zoom responders, the JSON and PHP tree outline (now on theme colors and repainted on a theme change).

Verified

Build, SwiftLint and the catalog check pass; 1,975 of 1,981 cases in the suites owning the changed types and the guard suites passed in one batch; 5 of the failures are load-timing and pass alone, the sixth was the separator color, fixed

Two tests follow the engine: the confusable-character underline is the theme's warning color, and the Vim UI suite scrolls to the Vim mode toggle, which the Editor Font section pushes below the fold of the default Settings window (scrollUntilHittable in UITestCase). The UI suite could not run locally, so CI is its first run. here. New tests cover the tree outline recoloring on a theme change and the grid line reaching the separators.

Not in this PR

• Docs half: docs(customization): document the rebuilt theme engine docs#5, merge after this.
• docs(claude-md): record that NSAppearance is the only native lever a theme has on chrome #2793 (chrome decision note) still to re-land.