feat: add missing DataTable component for admin pages

Fixes build issue caused by missing data-table import in:
- src/app/admin/exam-proctoring/page.tsx
- src/app/admin/signage/page.tsx

The component provides reusable table interface with search, sorting, and actions for all admin interfaces.

src/components/data-table.tsx

@@ -0,0 +1,148 @@
+"use client"
+
+import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "./ui/table"
+import { Button } from "./ui/button"
+import { Input } from "./ui/input"
+
+interface Column {
+  key: string
+  label: string
+  sortable?: boolean
+  width?: string
+  render?: (value: any, row: any) => React.ReactNode
+}
+
+interface Action {
+  label: string
+  onClick: (row: any) => void
+  variant?: "default" | "destructive" | "outline" | "secondary" | "ghost" | "link"
+}
+
+interface DataTableProps {
+  columns: Column[]
+  data: any[]
+  isLoading?: boolean
+  searchPlaceholder?: string
+  searchableColumns?: string[]
+  actions?: Action[]
+}
+
+export function DataTable({
+  columns,
+  data,
+  isLoading,
+  searchPlaceholder = "Search...",
+  searchableColumns = [],
+  actions = []
+}: DataTableProps) {
+  const [searchTerm, setSearchTerm] = useState("")
+  const [sortConfig, setSortConfig] = useState<{ key: string; direction: "asc" | "desc" } | null>(null)
+
+  const filteredData = data.filter((row) => {
+    if (!searchTerm) return true
+    return searchableColumns.some((key) => {
+      const value = row[key]
+      return value?.toString().toLowerCase().includes(searchTerm.toLowerCase())
+    })
+  })
+
+  const sortedData = [...filteredData].sort((a, b) => {
+    if (!sortConfig) return 0
+    const aValue = a[sortConfig.key]
+    const bValue = b[sortConfig.key]
+    
+    if (aValue < bValue) return sortConfig.direction === "asc" ? -1 : 1
+    if (aValue > bValue) return sortConfig.direction === "asc" ? 1 : -1
+    return 0
+  })
+
+  const handleSort = (key: string) => {
+    setSortConfig(current => {
+      if (current?.key === key) {
+        return { key, direction: current.direction === "asc" ? "desc" : "asc" }
+      }
+      return { key, direction: "asc" }
+    })
+  }
+
+  return (
+    <div className="space-y-4">
+      {searchableColumns.length > 0 && (
+        <Input
+          placeholder={searchPlaceholder}
+          value={searchTerm}
+          onChange={(e) => setSearchTerm(e.target.value)}
+          className="max-w-sm"
+        />
+      )}
+      
+      <Table>
+        <TableHeader>
+          <TableRow>
+            {columns.map((column) => (
+              <TableHead 
+                key={column.key} 
+                className={column.width ? `w-[${column.width}]` : ""}
+                onClick={column.sortable ? () => handleSort(column.key) : undefined}
+                style={{ cursor: column.sortable ? "pointer" : "default" }}
+              >
+                <div className="flex items-center gap-2">
+                  {column.label}
+                  {column.sortable && (
+                    <span className="text-xs text-gray-400">
+                      {sortConfig?.key === column.key && sortConfig.direction === "asc" ? "↑" : "↓"}
+                    </span>
+                  )}
+                </div>
+              </TableHead>
+            ))}
+            {actions.length > 0 && (
+              <TableHead className="w-[100px]">Actions</TableHead>
+            )}
+          </TableRow>
+        </TableHeader>
+        <TableBody>
+          {isLoading ? (
+            <TableRow>
+              <TableCell colSpan={columns.length + (actions.length > 0 ? 1 : 0)} className="text-center py-8">
+                Loading...
+              </TableCell>
+            </TableRow>
+          ) : sortedData.length === 0 ? (
+            <TableRow>
+              <TableCell colSpan={columns.length + (actions.length > 0 ? 1 : 0)} className="text-center py-8 text-gray-500">
+                No data found
+              </TableCell>
+            </TableRow>
+          ) : (
+            sortedData.map((row, index) => (
+              <TableRow key={index}>
+                {columns.map((column) => (
+                  <TableCell key={column.key}>
+                    {column.render ? column.render(row[column.key], row) : row[column.key]}
+                  </TableCell>
+                ))}
+                {actions.length > 0 && (
+                  <TableCell>
+                    <div className="flex gap-2">
+                      {actions.map((action, actionIndex) => (
+                        <Button
+                          key={actionIndex}
+                          size="sm"
+                          variant={action.variant || "outline"}
+                          onClick={() => action.onClick(row)}
+                        >
+                          {action.label}
+                        </Button>
+                      ))}
+                    </div>
+                  </TableCell>
+                )}
+              </TableRow>
+            ))
+          )}
+        </TableBody>
+      </Table>
+    </div>
+  )
+}