diff --git a/client/src/App.js b/client/src/App.js
index 1306eb3..7db0e2b 100644
--- a/client/src/App.js
+++ b/client/src/App.js
@@ -1,10 +1,11 @@
 import React, { Component } from 'react';
 import { ThemeProvider, createTheme } from '@mui/material/styles';
 import CssBaseline from '@mui/material/CssBaseline';
-import { Container, AppBar, Toolbar, Typography, Button, Box } from '@mui/material';
+import { Container, AppBar, Toolbar, Typography, Button, Box, Tabs, Tab } from '@mui/material';
 import LoginIcon from '@mui/icons-material/Login';
 import DashboardIcon from '@mui/icons-material/Dashboard';
 import DownloadIcon from '@mui/icons-material/Download';
+import TableChart from '@mui/icons-material/TableChart';
 import AuthService from './services/AuthService';
 import DataViewer from './components/DataViewer';
 import Login from './components/Login';
@@ -29,6 +30,7 @@ class App extends Component {
       user: null,
       loading: true,
       exportData: null, // { selectedMonth, canExport, onExport }
+      currentView: 'dashboard', // 'dashboard' or 'tables'
     };
     this.authService = new AuthService();
   }
@@ -77,8 +79,12 @@ class App extends Component {
     this.setState({ exportData });
   };
 
+  handleViewChange = (event, newValue) => {
+    this.setState({ currentView: newValue });
+  };
+
   render() {
-    const { isAuthenticated, user, loading } = this.state;
+    const { isAuthenticated, user, loading, currentView } = this.state;
 
     if (loading) {
       return (
@@ -99,7 +105,7 @@ class App extends Component {
         <AppBar position="static">
           <Toolbar>
             <DashboardIcon sx={{ mr: { xs: 1, sm: 2 } }} />
-            <Typography variant="h6" component="div" sx={{ flexGrow: 1 }}>
+            <Typography variant="h6" component="div" sx={{ mr: 3 }}>
               FibDash
             </Typography>
             {isAuthenticated && user && (
@@ -107,12 +113,42 @@ class App extends Component {
                 <Typography 
                   variant="body2" 
                   sx={{ 
-                    mr: { xs: 1, sm: 2 }, 
+                    mr: { xs: 2, sm: 3 }, 
                     display: { xs: 'none', sm: 'block' } 
                   }}
                 >
                   Willkommen, {user.name}
                 </Typography>
+                <Tabs
+                  value={currentView}
+                  onChange={this.handleViewChange}
+                  sx={{ 
+                    mr: 'auto',
+                    '& .MuiTab-root': { 
+                      color: 'rgba(255, 255, 255, 0.7)',
+                      minHeight: 48,
+                      '&.Mui-selected': {
+                        color: 'white'
+                      }
+                    },
+                    '& .MuiTabs-indicator': {
+                      backgroundColor: 'white'
+                    }
+                  }}
+                >
+                  <Tab 
+                    icon={<DashboardIcon />} 
+                    label="Dashboard" 
+                    value="dashboard"
+                    sx={{ minHeight: 48 }}
+                  />
+                  <Tab 
+                    icon={<TableChart />} 
+                    label="Stammdaten" 
+                    value="tables"
+                    sx={{ minHeight: 48 }}
+                  />
+                </Tabs>
                 {this.state.exportData && (
                   <Button 
                     color="inherit" 
@@ -153,7 +189,12 @@ class App extends Component {
         <Box sx={{ height: 'calc(100vh - 64px)', display: 'flex', flexDirection: 'column' }}>
           <Container maxWidth={false} sx={{ mt: 4, flex: 1, minHeight: 0, display: 'flex', flexDirection: 'column', width: '100%' }}>
             {isAuthenticated ? (
-              <DataViewer user={user} onUpdateExportData={this.updateExportData} />
+              <DataViewer 
+                user={user} 
+                onUpdateExportData={this.updateExportData}
+                currentView={currentView}
+                onViewChange={this.handleViewChange}
+              />
             ) : (
               <Login onLogin={this.handleLogin} />
             )}
diff --git a/client/src/components/DataViewer.js b/client/src/components/DataViewer.js
index bf4aa2a..e3f734c 100644
--- a/client/src/components/DataViewer.js
+++ b/client/src/components/DataViewer.js
@@ -7,7 +7,6 @@ import {
 import AuthService from '../services/AuthService';
 import SummaryHeader from './SummaryHeader';
 import TransactionsTable from './TransactionsTable';
-import Navigation from './Navigation';
 import Dashboard from './Dashboard';
 import TableManagement from './TableManagement';
 
@@ -21,7 +20,7 @@ class DataViewer extends Component {
       summary: null,
       loading: true,
       error: null,
-      currentView: 'dashboard', // 'dashboard' or 'tables'
+
     };
     this.authService = new AuthService();
   }
@@ -114,21 +113,17 @@ class DataViewer extends Component {
     if (this.props.onUpdateExportData) {
       this.props.onUpdateExportData({
         selectedMonth,
-        canExport: !!selectedMonth && !this.state.loading && this.state.currentView === 'dashboard',
+        canExport: !!selectedMonth && !this.state.loading && this.props.currentView === 'dashboard',
         onExport: this.downloadDatev
       });
     }
   };
 
-  handleViewChange = (event, newView) => {
-    this.setState({ currentView: newView });
-    // Update export data when view changes
-    this.updateExportData();
-  };
+
 
   render() {
-    const { months, selectedMonth, transactions, summary, loading, error, currentView } = this.state;
-    const { user } = this.props;
+    const { months, selectedMonth, transactions, summary, loading, error } = this.state;
+    const { user, currentView } = this.props;
 
     if (loading && !transactions.length) {
       return (
@@ -148,11 +143,6 @@ class DataViewer extends Component {
 
     return (
       <Box sx={{ height: '100vh', display: 'flex', flexDirection: 'column' }}>
-        <Navigation 
-          currentView={currentView}
-          onViewChange={this.handleViewChange}
-        />
-        
         {currentView === 'dashboard' ? (
           <>
             <Box sx={{ flexShrink: 0 }}>
diff --git a/client/src/components/admin/BUTable.js b/client/src/components/admin/BUTable.js
index af58f02..aaf91e3 100644
--- a/client/src/components/admin/BUTable.js
+++ b/client/src/components/admin/BUTable.js
@@ -72,7 +72,7 @@ class BUTable extends Component {
       formData: bu ? {
         bu: bu.bu,
         name: bu.name,
-        vst: bu.vst || '',
+        vst: bu.vst !== null && bu.vst !== undefined ? bu.vst.toString() : '',
       } : {
         bu: '',
         name: '',
@@ -102,13 +102,20 @@ class BUTable extends Component {
     });
   };
 
+  isFormValid = () => {
+    const { formData } = this.state;
+    return formData.bu.trim() !== '' && 
+           formData.name.trim() !== '' && 
+           formData.vst !== '';
+  };
+
   handleSave = async () => {
     const { editingBU, formData } = this.state;
     
     // Convert vst to number or null
     const payload = {
       ...formData,
-      vst: formData.vst ? parseFloat(formData.vst) : null,
+      vst: formData.vst !== '' ? parseFloat(formData.vst) : null,
     };
     
     try {
@@ -220,7 +227,7 @@ class BUTable extends Component {
                   <TableCell>{bu.bu}</TableCell>
                   <TableCell>{bu.name}</TableCell>
                   <TableCell align="right">
-                    {bu.vst ? `${bu.vst}%` : '-'}
+                    {bu.vst !== null && bu.vst !== undefined ? `${bu.vst}%` : '-'}
                   </TableCell>
                   <TableCell align="right">
                     <IconButton
@@ -287,7 +294,11 @@ class BUTable extends Component {
           </DialogContent>
           <DialogActions>
             <Button onClick={this.handleCloseDialog}>Abbrechen</Button>
-            <Button onClick={this.handleSave} variant="contained">
+            <Button 
+              onClick={this.handleSave} 
+              variant="contained"
+              disabled={!this.isFormValid()}
+            >
               Speichern
             </Button>
           </DialogActions>
diff --git a/src/routes/admin.js b/src/routes/admin.js
index 7ce59a8..b5a404b 100644
--- a/src/routes/admin.js
+++ b/src/routes/admin.js
@@ -199,7 +199,7 @@ router.post('/buchungsschluessel', authenticateToken, async (req, res) => {
   try {
     await executeQuery(
       'INSERT INTO fibdash.BU (bu, name, vst) VALUES (@bu, @name, @vst)',
-      { bu, name, vst: vst || null }
+      { bu, name, vst: vst !== undefined && vst !== '' ? vst : null }
     );
     res.json({ message: 'Buchungsschlüssel erfolgreich erstellt' });
   } catch (error) {
@@ -224,7 +224,7 @@ router.put('/buchungsschluessel/:id', authenticateToken, async (req, res) => {
   try {
     await executeQuery(
       'UPDATE fibdash.BU SET bu = @bu, name = @name, vst = @vst WHERE id = @id',
-      { bu, name, vst: vst || null, id }
+      { bu, name, vst: vst !== undefined && vst !== '' ? vst : null, id }
     );
     res.json({ message: 'Buchungsschlüssel erfolgreich aktualisiert' });
   } catch (error) {