feat(desktop): add neutral accent color

[thomaspblock]

Summary

• Add a Neutral option to the desktop Appearance accent color picker.
• Resolve the neutral accent from the active theme foreground/background variables so it follows each theme's default text color.
• Update the neutral swatch preview and checkmark contrast for light and dark themes.

Test plan

• pnpm exec biome check src/shared/theme/ThemeProvider.tsx src/features/settings/ui/SettingsPanels.tsx
• Pre-commit checks passed during commit: desktop check, desktop Tauri fmt, mobile check, Rust fmt, and web check.

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[thomaspblock]