feat(settings): create appearance tab with themes and display controls

[tyulyukov]

Summary

• Created AppearanceSettingsPanel with Telegram-style theme preview cards showing color palettes and layout
• Reorganized settings into dedicated tabs: General, Appearance, Connections, and Archive
• Added conversation width control (narrow/comfortable/wide) with CSS variable binding to --chat-column-max-width
• Added display toggles for reduce motion and ambient grain
• Replaced static max-w-208 / max-w-3xl width constraints across chat UI with dynamic CSS variable
• Added appearance.ts utility for theme preview palettes and conversation width configuration
• Added settingsRestore.ts for the restore-to-defaults logic (moved from SettingsPanels.tsx)
• Updated settings navigation sidebar with Appearance tab and new icon
• Added browser test verifying theme selection, conversation width, and display settings persist to localStorage

Testing

• Verify Appearance tab loads with theme gallery organized by groups
• Click each theme and confirm the preview updates and selection badge appears
• Select conversation width option and verify the chat column width adjusts dynamically
• Toggle "Reduce motion" and "Ambient grain" and confirm settings persist to localStorage (marcode:client-settings:v1)
• Verify General tab no longer shows theme picker, time format, or diff wrapping controls
• Check that all three conversation width options display correct preview widths
• Confirm reset buttons appear and clear appearance settings back to defaults