feat: Unify navigation with consistent header and centered bottom menu

- Removed duplicate MobileNav AppBar from desktop view
- Added unified header bar across all screen sizes (mobile + desktop)
- Positioned connection status on left, user menu on right in header
- Centered bottom navigation menu on desktop (30% width with 20px margin)
- Hidden floating voice button on desktop (using center button instead)
- Updated voice command button to pink color (#FF69B4) matching design
- Added rounded corners to desktop bottom navigation

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

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

maternal-web/components/voice/VoiceFloatingButton.tsx

@@ -323,7 +323,7 @@ export function VoiceFloatingButton() {
 
   return (
     <>
-      {/* Floating button positioned in bottom-right - Hidden on mobile since we have TabBar center button */}
+      {/* Floating button positioned in bottom-right - Hidden on desktop since we have TabBar center button */}
       <Tooltip title="Voice Command (Beta)" placement="left">
         <Fab
           color="primary"
@@ -335,7 +335,7 @@ export function VoiceFloatingButton() {
             bottom: 24,
             right: 24,
             zIndex: 1000,
-            display: { xs: 'none', md: 'flex' }, // Hide on mobile (xs) and small screens, show on medium+
+            display: 'none', // Hidden - using TabBar center button instead
           }}
         >
           <MicIcon />