refactor: migrate theme system to native macOS patterns

[datlechin]

Summary

• Semantic colors by default: UI/sidebar/toolbar colors now resolve to native NSColor equivalents (labelColor, separatorColor, controlBackgroundColor, etc.) instead of hardcoded hex. They adapt automatically to light/dark mode, system accent color, and high contrast settings. Custom themes can still override with hex values.
• Native typography: Replaced ThemeEngine.shared.activeTheme.typography.* across 65 files with SwiftUI semantic text styles (.body, .caption, .subheadline, .callout, .title3, .title2) for proper Dynamic Type and accessibility support.
• Removed layout customization: Deleted ThemeTypography, ThemeSpacing, ThemeCornerRadius, ThemeIconSizes, ThemeRowHeights, ThemeAnimations structs and the Layout tab from the theme editor. Spacing/radius/icon values replaced with literals.
• Native empty states: EmptyStateView now wraps ContentUnavailableView (macOS 14+).
• Minor fixes: removed deprecated UserDefaults.synchronize(), fixed VimRegister pasteboard side-effect in didSet (now explicit syncToPasteboard()), fixed AboutWindowController panel memory retention.

93 files changed, 623 insertions, 1190 deletions.

Test plan

• Switch between Default Light, Default Dark, Dracula, Nord themes — verify visual correctness
• Toggle macOS light/dark mode in Auto appearance mode — UI colors should adapt
• Change system accent color — selection highlights should follow
• Enable high contrast (Accessibility > Display) — text/borders should become more distinct
• Open Settings > Appearance — verify only Fonts + Colors tabs (no Layout)
• Duplicate a built-in theme, edit colors, verify "Reset to Default" button works
• Import an old-format theme JSON (with typography/spacing sections) — should load without errors
• Verify theme preview cards show correct sidebar strips for all themes
• Check empty states: AI Chat panel, Structure tab (foreign keys, indexes, check constraints)
• Vim mode: yank text (yy), paste in external app — pasteboard sync should work
• Open and close About window — verify no memory growth