DBSyn Phase 61.1 — Light and Dark Appearance

Appearance setting under Settings → General: System, Light, Dark; stored in preferences.appearance.
Appearance selection applies to the application window and preferences; System follows macOS automatically.
Replaced fixed light-mode toolbar, navigator, header, and border colors with semantic macOS colors.
Removed light-only overrides from settings and splash screen; splash gradient uses adaptive system colors.
SQL NSTextView and native NSTableView results use light/dark AppKit appearances and semantic text/background colors, refreshed when appearance changes.
Existing database, charts, SQL editor, backup catalog and administration features retained.

Validation: Swift syntax parsed using swiftc -frontend -parse; full macOS Xcode build and visual testing are not available in this environment.