feat: initialize frontend project structure with Material UI and DataGrid implementation

frontend/src/components/Header.tsx

@@ -14,6 +14,7 @@ const Header: React.FC = () => {
         borderBottom: 1, 
         borderColor: 'divider',
         bgcolor: darkMode ? 'rgba(16, 24, 48, 0.95)' : 'rgba(255, 255, 255, 0.95)',
+        color: darkMode ? '#f1f5f9' : '#1e293b',
         backdropFilter: 'blur(10px)',
         zIndex: (theme) => theme.zIndex.drawer + 1
       }}
@@ -23,10 +24,10 @@ const Header: React.FC = () => {
           MARIAVEL
         </Typography>
         <Box sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
-          <IconButton onClick={toggleDarkMode} color="inherit">
+          <IconButton onClick={toggleDarkMode} color="inherit" sx={{ opacity: 0.8, '&:hover': { opacity: 1 } }}>
             {darkMode ? <Brightness7 /> : <Brightness4 />}
           </IconButton>
-          <IconButton color="inherit">
+          <IconButton color="inherit" sx={{ opacity: 0.8, '&:hover': { opacity: 1 } }}>
             <Settings />
           </IconButton>
         </Box>