feat: P2 strankovani - PaginationHelper, Pagination komponenta, integrace do 4 modulu

- PaginationHelper.php: parseParams() + paginate() - DRY backend pagination logika
- Pagination.jsx: frontend strankovaci komponenta (prev/next/cisla/info)
- CSS: .admin-pagination styly v admin.css
- Refaktor handleru: offers, orders, invoices, projects pouzivaji PaginationHelper
- Default 25 zaznamu na stranku (misto 500), max 500
- Frontend: page state + reset na search/filter zmenu
- useListData: pagination data v response

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

api/includes/PaginationHelper.php

@@ -0,0 +1,82 @@
+<?php
+
+/**
+ * Pagination helper - extrakce spolecne logiky pro strankovani seznamu.
+ */
+
+declare(strict_types=1);
+
+class PaginationHelper
+{
+    private const DEFAULT_PER_PAGE = 25;
+    private const MAX_PER_PAGE = 500;
+
+    /**
+     * Nacte pagination parametry z GET requestu.
+     *
+     * @return array{page: int, per_page: int, sort: string, order: string, search: string}
+     */
+    public static function parseParams(array $sortMap, string $defaultSort = 'created_at'): array
+    {
+        $sort = $_GET['sort'] ?? $defaultSort;
+        $order = strtoupper($_GET['order'] ?? 'DESC') === 'ASC' ? 'ASC' : 'DESC';
+        $page = max(1, (int) ($_GET['page'] ?? 1));
+        $perPage = min(self::MAX_PER_PAGE, max(1, (int) ($_GET['per_page'] ?? self::DEFAULT_PER_PAGE)));
+        $search = trim($_GET['search'] ?? '');
+
+        if (!isset($sortMap[$sort])) {
+            errorResponse('Neplatný parametr řazení', 400);
+        }
+
+        return [
+            'page' => $page,
+            'per_page' => $perPage,
+            'sort' => $sortMap[$sort],
+            'order' => $order,
+            'search' => $search ? mb_substr($search, 0, 100) : '',
+        ];
+    }
+
+    /**
+     * Spusti COUNT + SELECT dotazy s pagination a vrati vysledek.
+     *
+     * @param PDO $pdo
+     * @param string $countSql - COUNT(*) dotaz
+     * @param string $dataSql - SELECT dotaz (bez LIMIT/OFFSET)
+     * @param array $params - parametry pro prepared statement
+     * @param array{page: int, per_page: int, sort: string, order: string} $pagination
+     * @return array{items: array, pagination: array}
+     */
+    public static function paginate(
+        PDO $pdo,
+        string $countSql,
+        string $dataSql,
+        array $params,
+        array $pagination
+    ): array {
+        $page = $pagination['page'];
+        $perPage = $pagination['per_page'];
+
+        $stmt = $pdo->prepare($countSql);
+        $stmt->execute($params);
+        $total = (int) $stmt->fetchColumn();
+
+        $offset = ($page - 1) * $perPage;
+        $totalPages = (int) ceil($total / $perPage);
+
+        $sql = "{$dataSql} LIMIT {$perPage} OFFSET {$offset}";
+        $stmt = $pdo->prepare($sql);
+        $stmt->execute($params);
+        $items = $stmt->fetchAll();
+
+        return [
+            'items' => $items,
+            'pagination' => [
+                'total' => $total,
+                'page' => $page,
+                'per_page' => $perPage,
+                'total_pages' => $totalPages,
+            ],
+        ];
+    }
+}