refactor(settings)!: draw default themes in macOS system colors

[datlechin]

Why

After #2791 the app stopped looking like macOS. A wrapper recolored the text in every content pane, the default themes used fixed hex for the editor and status colors, the grid painted its selection as a 28% wash under white text, and baked colors ignored Accent, Highlight and Increase Contrast. Every theme had to declare all 43 slots, so "default = native" could not be expressed.

Change

• Default Light and Default Dark draw the editor and grid in system colors, with Xcode's default syntax colors and its high-contrast set under Increase Contrast.
• A theme colors only code views and the data grid. Every slot is optional (theme format 3); a missing one draws the system color.
• Other panes are back on system colors. The grid uses AppKit's own selection, header and separator colors.
• Settings > Appearance: Light and Dark theme pickers, a Custom Themes list and an edit sheet.
• A custom theme is saved to, renamed in and deleted from its own file, and a file changed on disk is never overwritten.

Verified

Source-scan guards fail on any theme read, literal color or hidden pane background outside the allowed files. A default-theme selected row matches a stock NSTableRowView pixel for pixel. A UI test drives create, rename, reset and delete.

Screenshots

	Before	After
Light
Dark
Settings

Dracula colors only the editor and the grid:

Not in this PR

• Integer literals in the live editor take the string color, before and after this change; TreeSitterClient.quickHighlight colors them as numbers.
• The UI test defaults domain outlives a test: UITestCase clears it from inside the runner's sandbox container, so it never reaches the app's com.TablePro.uitest suite.
• Docs: docs(customization): document native default themes and theme scope docs#14, merge after this.