feat: Add dark/light mode theme switching with dynamic favicons

Implemented comprehensive dark/light mode support throughout the app:

- Added ColorSchemeScript to layout for auto-detection of system preference
- Updated MantineProvider to use 'auto' color scheme (respects system)
- Updated theme.ts with dynamic Paper component styles based on color scheme
- Created ThemeToggle component with sun/moon icons
- Added toggle to desktop sidebar navigation
- Created theme-specific favicons (favicon-light.svg, favicon-dark.svg)
- Made ThoughtGalaxy 3D visualization theme-aware:
  - Dynamic node colors based on theme
  - Theme-aware lighting intensity
  - Theme-aware link colors
  - Theme-aware text labels
- Added comprehensive Playwright tests for theme functionality
- Theme preference persists via localStorage

Tested manually with Playwright MCP:
- ✅ Theme toggle switches between light and dark modes
- ✅ Theme persists across page reloads
- ✅ Both modes render correctly with appropriate colors
- ✅ Icons change based on current theme (sun/moon)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/Navigation/DesktopSidebar.tsx

@@ -13,6 +13,7 @@ import { IconMessageCircle, IconEdit, IconChartBubbleFilled } from '@tabler/icon
 import { useSelector } from '@xstate/react';
 import { useAppMachine } from '@/hooks/useAppMachine';
 import { UserMenu } from '@/components/UserMenu';
+import { ThemeToggle } from '@/components/ThemeToggle';
 
 export function DesktopSidebar() {
   const actor = useAppMachine();
@@ -111,6 +112,9 @@ export function DesktopSidebar() {
 
         <Divider my="md" color="#373A40" />
 
+        {/* Theme Toggle */}
+        <ThemeToggle />
+
         {/* User Menu - styled like other nav items */}
         <UserMenu showLabel={true} />