fix: pwa ios safari safe area inset for bottom navigation

- Add viewport-fit=cover meta tag for PWA fullscreen display on iOS
- Implement safe-area-inset-bottom for mobile bottom navigation bar
- Update PageLayout main content padding to avoid iOS home indicator overlap
- Add CSS utility class pb-safe-bottom for PWA safe area handling
- Ensure proper PWA layout on iOS devices with notch and home indicator
- Set minimum navigation height including safe area for PWA compatibility

src/components/MobileBottomNav.tsx

@@ -58,6 +58,7 @@ const MobileBottomNav = ({ activePath }: MobileBottomNavProps) => {
         /* 紧贴视口底部，同时在内部留出安全区高度 */
         bottom: 0,
         paddingBottom: 'env(safe-area-inset-bottom)',
+        minHeight: 'calc(3.5rem + env(safe-area-inset-bottom))',
       }}
     >
       <ul className='flex items-center'>