feat(settings): rebuild the theme system and theme every content surface

[datlechin]

Why

A theme reached only part of the window. Many content views drew system colors, pane bars drew a system band over a themed window, and a slot a theme left out could clash with its background. Edits reached open windows only through Settings, and four themes shipped.

Change

• New TableProTheme package: the slot registry with fallback rules, the file format, the resolver and VS Code theme import. The engine follows the catalog and previews unsaved edits live.
• Every content surface follows the theme. On macOS 26 the four content windows show it through the titlebar, and bars drop their material over a themed pane.
• A Theme Editor window with a live preview, undo and Revert, plus a Themes list with drag and drop and Move to Trash. 12 new built-in themes.
• Custom themes and the light and dark choice sync over iCloud. Appearance mode is now per Mac, so a Mac on 0.81 reads System.

Verified

Unit plan, TableProCore package tests and the related UI flows pass. Changed on purpose: AppearanceSettingsUITests, EditorThemeBridgeTests, ThemeContrastTests (light themes held to WCAG 3:1), EditorTabStripTrackThemeTests, HighlightedSQLTextViewLayoutTests, DetailSurfaceThemeTests, ThemeFileImporterTests.

Screenshots

Before and after, Dracula:

New light theme (Solarized Light):

Appearance pane, before and after:

Theme Editor:

Not in this PR

• The grid header's filter funnel turns white over an unfocused column selection (older bug).
• DDLTextView and TriggerEditorView ignore the editor font.
• The Settings window is resizable, against HIG.
• Docs: docs(customization): document the Theme Editor, import and theme sync docs#29, merges after this one.