Improve voice command UX and add desktop home navigation

Voice command improvements:
- Auto-start listening when voice dialog opens (removes extra tap/click)
- Added Activity option to unknown intent dialog
- Users can now speak immediately after clicking the mic button

Desktop navigation:
- Added Home icon button in top bar/header for quick navigation to main page
- Positioned between app title and user avatar

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

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

maternal-web/components/layouts/MobileNav/MobileNav.tsx

@@ -63,6 +63,14 @@ export const MobileNav = () => {
           <Typography variant="h6" component="div" sx={{ flexGrow: 1, color: 'primary.main', fontWeight: 600 }}>
             Maternal
           </Typography>
+          <IconButton
+            color="primary"
+            aria-label="home"
+            onClick={() => router.push('/')}
+            sx={{ mr: 1 }}
+          >
+            <Home />
+          </IconButton>
           <Avatar sx={{ bgcolor: 'primary.main' }}>U</Avatar>
         </Toolbar>
       </AppBar>