feat: Add database explorer page with SQLite management

- Add /database page with table explorer, query executor and stats
- Implement MCP tools: list_tables, get_table_schema, get_table_data,
  get_database_stats, execute_query
- Add database API endpoints with security (SELECT only)
- Add database icon to toolbar

frontend/src/components/Toolbar.vue

@@ -95,6 +95,14 @@ onMounted(() => {
           <path d="M12 2C6.5 2 2 6.5 2 12s4.5 10 10 10c.93 0 1.82-.13 2.67-.36"/>
         </svg>
       </RouterLink>
+
+      <RouterLink to="/database" class="toolbar-btn" :class="{ active: route.path === '/database' }" title="Database">
+        <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+          <ellipse cx="12" cy="5" rx="9" ry="3"/>
+          <path d="M3 5v14c0 1.66 4 3 9 3s9-1.34 9-3V5"/>
+          <path d="M3 12c0 1.66 4 3 9 3s9-1.34 9-3"/>
+        </svg>
+      </RouterLink>
     </div>
 
     <div class="toolbar-divider"></div>