feat: Implement loan management features including fetching, creating, and deleting loans

- Added database functions for managing loans: getLoans, getUserLoans, deleteLoan, and createLoan.
- Updated frontend to include new Form4 for displaying user loans and handling loan deletions.
- Replaced Form3 with Form4 in App component.
- Enhanced Form1 to set borrowing dates and fetch available items.
- Improved Form2 to display borrowable items and allow selection for loans.
- Introduced utility functions for handling loan creation and deletion in userHandler.
- Added event listeners for local storage updates to keep UI in sync.
- Updated fetchData utility to retrieve loans and user loans from the backend.

frontend/src/utils/userHandler.ts

@@ -0,0 +1,96 @@
+import { myToast } from "./toastify";
+import Cookies from "js-cookie";
+
+export const handleDeleteLoan = async (loanID: number): Promise<boolean> => {
+  try {
+    const response = await fetch(
+      `http://localhost:8002/api/deleteLoan/${loanID}`,
+      {
+        method: "DELETE",
+        headers: {
+          Authorization: `Bearer ${Cookies.get("token") || ""}`,
+        },
+      }
+    );
+
+    if (!response.ok) {
+      myToast("Fehler beim Löschen der Ausleihe", "error");
+      return false;
+    }
+
+    const raw = localStorage.getItem("userLoans");
+    let current: Array<{ id: number }> = [];
+
+    try {
+      const parsed = raw ? JSON.parse(raw) : [];
+      current = Array.isArray(parsed) ? parsed : [];
+    } catch {
+      current = [];
+    }
+
+    const updated = current.filter(
+      (loan) => Number(loan.id) !== Number(loanID)
+    );
+    localStorage.setItem("userLoans", JSON.stringify(updated));
+
+    myToast("Ausleihe erfolgreich gelöscht!", "success");
+    return true;
+  } catch (error) {
+    console.error("Error deleting loan:", error);
+    myToast("Fehler beim löschen der Ausleihe", "error");
+    return false;
+  }
+};
+
+// Parse existing cookie and coerce to numbers
+let removeArr: number[] = (() => {
+  try {
+    const raw = Cookies.get("removeArr");
+    const parsed = raw ? JSON.parse(raw) : [];
+    return Array.isArray(parsed)
+      ? parsed.map((v) => Number(v)).filter((n) => Number.isFinite(n))
+      : [];
+  } catch {
+    return [];
+  }
+})();
+
+const rawCookies = Cookies.withConverter({
+  write: (value: string) => value, // store raw JSON
+});
+
+export const addToRemove = (itemID: number) => {
+  if (!Number.isFinite(itemID)) return;
+  if (!removeArr.includes(itemID)) {
+    removeArr.push(itemID);
+    rawCookies.set("removeArr", JSON.stringify(removeArr));
+  }
+};
+
+export const rmFromRemove = (itemID: number) => {
+  removeArr = removeArr.filter((item) => item !== itemID);
+  rawCookies.set("removeArr", JSON.stringify(removeArr));
+};
+
+export const createLoan = async (startDate: string, endDate: string) => {
+  const items = removeArr;
+  const response = await fetch("http://localhost:8002/api/createLoan", {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json",
+      Authorization: `Bearer ${Cookies.get("token") || ""}`,
+    },
+    body: JSON.stringify({ items, startDate, endDate }),
+  });
+
+  if (!response.ok) {
+    myToast("Fehler beim Erstellen der Ausleihe", "error");
+    return false;
+  }
+
+  // Clear selection on success
+  removeArr = [];
+  Cookies.set("removeArr", "[]");
+  myToast("Ausleihe erfolgreich erstellt!", "success");
+  return true;
+};