feat: add Sign PDF tool (draw/type/upload signatures, place on a PDF) (#370)

Draw, type, or upload a signature and place resizable/rotatable copies across PDF pages; output flattened server-side with PyMuPDF. Visual electronic signature, not cryptographic. New interactive-sign display mode (pdf.js + Konva) and a custom docs-pool route.

tests/unit/web/signature-store.test.ts

@@ -0,0 +1,45 @@
+// @vitest-environment jsdom
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  addSignature,
+  deleteSignature,
+  listSignatures,
+  MAX_SIGNATURES,
+} from "@/lib/signature-store";
+
+// The jsdom env's native localStorage is a broken stub here (Node's experimental
+// Web Storage shadows it, missing methods), so back it with an in-memory map --
+// the same approach the other web unit tests use.
+const storageMap = new Map<string, string>();
+vi.stubGlobal("localStorage", {
+  getItem: vi.fn((key: string) => storageMap.get(key) ?? null),
+  setItem: vi.fn((key: string, val: string) => storageMap.set(key, val)),
+  removeItem: vi.fn((key: string) => storageMap.delete(key)),
+  clear: vi.fn(() => storageMap.clear()),
+  get length() {
+    return storageMap.size;
+  },
+  key: vi.fn((_i: number) => null),
+});
+
+describe("signature-store", () => {
+  beforeEach(() => localStorage.clear());
+
+  it("adds and lists signatures", () => {
+    addSignature("data:image/png;base64,AAAA");
+    expect(listSignatures()).toHaveLength(1);
+    expect(listSignatures()[0].dataUrl).toContain("base64,AAAA");
+  });
+
+  it("caps the library at MAX_SIGNATURES (drops oldest)", () => {
+    for (let i = 0; i < MAX_SIGNATURES + 3; i++) addSignature(`data:image/png;base64,S${i}`);
+    expect(listSignatures()).toHaveLength(MAX_SIGNATURES);
+    expect(listSignatures().some((s) => s.dataUrl.endsWith("S0"))).toBe(false);
+  });
+
+  it("deletes by id", () => {
+    const sig = addSignature("data:image/png;base64,BBBB");
+    deleteSignature(sig.id);
+    expect(listSignatures()).toHaveLength(0);
+  });
+});