fix: add logout button to vendor onboarding page

The onboarding page was a standalone page with no way to log out,
leaving users stuck. Added:
- Logout button in the header
- handleLogout() function to clear vendor tokens and redirect to login

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/vendor/js/onboarding.js

@@ -603,6 +603,35 @@ function vendorOnboarding(initialLang = 'en') {
             }
         },
 
+        // Logout handler
+        async handleLogout() {
+            console.log('🚪 Logging out from onboarding...');
+
+            // Get vendor code from URL
+            const path = window.location.pathname;
+            const segments = path.split('/').filter(Boolean);
+            const vendorCode = segments[0] === 'vendor' && segments[1] ? segments[1] : '';
+
+            try {
+                // Call logout API
+                await apiClient.post('/vendor/auth/logout');
+                console.log('✅ Logout API called successfully');
+            } catch (error) {
+                console.error('⚠️ Logout API error (continuing anyway):', error);
+            } finally {
+                // Clear vendor tokens only (not admin or customer tokens)
+                console.log('🧹 Clearing vendor tokens...');
+                localStorage.removeItem('vendor_token');
+                localStorage.removeItem('vendor_user');
+                localStorage.removeItem('currentUser');
+                localStorage.removeItem('vendorCode');
+                // Note: Do NOT use localStorage.clear() - it would clear admin/customer tokens too
+
+                console.log('🔄 Redirecting to login...');
+                window.location.href = `/vendor/${vendorCode}/login`;
+            }
+        },
+
         // Dark mode
         get dark() {
             return localStorage.getItem('dark') === 'true' ||