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/ThemeToggle.tsx

@@ -0,0 +1,35 @@
+'use client';
+
+import { ActionIcon, useMantineColorScheme, useComputedColorScheme } from '@mantine/core';
+import { IconSun, IconMoon } from '@tabler/icons-react';
+
+/**
+ * ThemeToggle Component
+ *
+ * Toggles between light and dark color schemes.
+ * Uses Mantine's built-in color scheme hooks for state management and persistence.
+ */
+export function ThemeToggle() {
+  const { setColorScheme } = useMantineColorScheme();
+  const computedColorScheme = useComputedColorScheme('light');
+
+  const toggleColorScheme = () => {
+    setColorScheme(computedColorScheme === 'dark' ? 'light' : 'dark');
+  };
+
+  return (
+    <ActionIcon
+      onClick={toggleColorScheme}
+      variant="subtle"
+      size="lg"
+      aria-label="Toggle color scheme"
+      title={`Switch to ${computedColorScheme === 'dark' ? 'light' : 'dark'} mode`}
+    >
+      {computedColorScheme === 'dark' ? (
+        <IconSun size={20} />
+      ) : (
+        <IconMoon size={20} />
+      )}
+    </ActionIcon>
+  );
+}