feat: drag-to-reorder table groups in zone tabs

Backend:
  - tables.py: PUT /api/tables/groups/reorder accepts an ordered list
    of group IDs and updates sort_order accordingly

Frontend (TablesConfigTab):
  - Zone tabs are now draggable for group tabs (not All/Ungrouped)
  - HTML5 drag-and-drop with visual drop-target highlight and a
    grab cursor; fires reorderGroups mutation on drop

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

manager_dashboard/src/pages/TablesConfigTab.jsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useState, useRef } from 'react'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import toast from 'react-hot-toast'
 import client from '../api/client'
@@ -43,6 +43,8 @@ export default function TablesPage() {
   const [activeTab, setActiveTab] = useState('all')   // 'all' | group.id
   const [selected, setSelected] = useState(new Set())
   const [anyHovered, setAnyHovered] = useState(false)
+  const dragGroupId = useRef(null)
+  const [dragOverId, setDragOverId] = useState(null)
 
   const { data: tables = [], isLoading } = useQuery({
     queryKey: ['tables-all', showInactive],
@@ -102,6 +104,12 @@ export default function TablesPage() {
     onError: () => toast.error('Σφάλμα'),
   })
 
+  const reorderGroups = useMutation({
+    mutationFn: (ids) => client.put('/api/tables/groups/reorder', ids),
+    onSuccess: invalidateGroups,
+    onError: () => toast.error('Σφάλμα αναδιάταξης'),
+  })
+
   // Filter tables for the active tab
   const visibleTables = activeTab === 'all'
     ? tables
@@ -156,23 +164,47 @@ export default function TablesPage() {
 
       {/* Zone tabs */}
       <div className="flex gap-0 flex-wrap border-b border-slate-200 px-4 flex-shrink-0">
-        {zoneTabs.map(tab => (
-          <button
-            key={tab.id}
-            onClick={() => { setActiveTab(tab.id); clearSelect() }}
-            className={`flex items-center gap-1.5 px-4 py-2.5 text-sm font-medium transition-colors border-b-2 -mb-px ${
-              activeTab === tab.id
-                ? 'border-sky-500 text-sky-600'
-                : 'border-transparent text-slate-500 hover:text-slate-700 hover:border-slate-300'
-            }`}
-          >
-            {tab.color && <span className="w-2 h-2 rounded-full shrink-0" style={{ background: tab.color }} />}
-            {tab.label}
-            <span className="ml-0.5 text-xs text-slate-400">
-              ({tab.id === 'all' ? tables.length : tab.id === 'ungrouped' ? tables.filter(t => !t.group_id).length : tables.filter(t => t.group_id === tab.id).length})
-            </span>
-          </button>
-        ))}
+        {zoneTabs.map(tab => {
+          const isGroup = tab.id !== 'all' && tab.id !== 'ungrouped'
+          const isDragOver = dragOverId === tab.id
+          return (
+            <button
+              key={tab.id}
+              onClick={() => { setActiveTab(tab.id); clearSelect() }}
+              draggable={isGroup}
+              onDragStart={isGroup ? () => { dragGroupId.current = tab.id } : undefined}
+              onDragOver={isGroup ? (e) => { e.preventDefault(); setDragOverId(tab.id) } : undefined}
+              onDragLeave={isGroup ? () => setDragOverId(null) : undefined}
+              onDrop={isGroup ? (e) => {
+                e.preventDefault()
+                setDragOverId(null)
+                const fromId = dragGroupId.current
+                if (!fromId || fromId === tab.id) return
+                const groupIds = groups.map(g => g.id)
+                const fromIdx = groupIds.indexOf(fromId)
+                const toIdx = groupIds.indexOf(tab.id)
+                if (fromIdx === -1 || toIdx === -1) return
+                const reordered = [...groupIds]
+                reordered.splice(fromIdx, 1)
+                reordered.splice(toIdx, 0, fromId)
+                reorderGroups.mutate(reordered)
+              } : undefined}
+              onDragEnd={isGroup ? () => { dragGroupId.current = null; setDragOverId(null) } : undefined}
+              className={`flex items-center gap-1.5 px-4 py-2.5 text-sm font-medium transition-colors border-b-2 -mb-px ${
+                activeTab === tab.id
+                  ? 'border-sky-500 text-sky-600'
+                  : 'border-transparent text-slate-500 hover:text-slate-700 hover:border-slate-300'
+              } ${isDragOver ? 'bg-sky-50 border-sky-300' : ''} ${isGroup ? 'cursor-grab active:cursor-grabbing' : ''}`}
+            >
+              {isGroup && <span className="text-slate-300 text-xs mr-0.5 select-none">⠿</span>}
+              {tab.color && <span className="w-2 h-2 rounded-full shrink-0" style={{ background: tab.color }} />}
+              {tab.label}
+              <span className="ml-0.5 text-xs text-slate-400">
+                ({tab.id === 'all' ? tables.length : tab.id === 'ungrouped' ? tables.filter(t => !t.group_id).length : tables.filter(t => t.group_id === tab.id).length})
+              </span>
+            </button>
+          )
+        })}
       </div>
 
       {/* Zone action bar (when viewing a specific zone) */}