feat: Move theme toggle to profile dropdown with icon-only SegmentedControl

Changes:
- Moved theme toggle from separate DesktopSidebar component into UserMenu dropdown
- Replaced simple light/dark toggle with SegmentedControl offering three options:
  - Light (sun icon)
  - Dark (moon icon)
  - System/Auto (desktop icon)
- Uses icon-only labels for compact display in dropdown menu
- Defaults to 'auto' mode (respects system preference) as configured in layout.tsx
- Removed standalone ThemeToggle component from DesktopSidebar

Benefits:
- Cleaner navigation UI with one less separate control
- Better UX with system preference option
- More compact dropdown menu layout

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

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

components/Navigation/DesktopSidebar.tsx

@@ -13,7 +13,6 @@ 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';
 import styles from './DesktopSidebar.module.css';
 
 export function DesktopSidebar() {
@@ -106,10 +105,7 @@ export function DesktopSidebar() {
 
         <Divider my="md" />
 
-        {/* Theme Toggle */}
-        <ThemeToggle />
-
-        {/* User Menu - styled like other nav items */}
+        {/* User Menu - styled like other nav items, now includes theme toggle */}
         <UserMenu showLabel={true} />
 
         {/* Development state panel */}