test: add unit tests for timestamp PDF feature

Add 31 tests across 3 test files covering:
- TSA preset configuration (uniqueness, valid URLs, known providers)
- Timestamp PDF page logic (file validation, output naming, UI state, drop zone)
- TimestampNode workflow node (category, icon, description, presets)

src/tests/timestamp-tsa.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect } from 'vitest';
+import {
+  TIMESTAMP_TSA_PRESETS,
+  type TimestampTsaPreset,
+} from '@/js/config/timestamp-tsa';
+
+describe('Timestamp TSA Presets', () => {
+  it('should be a non-empty array', () => {
+    expect(Array.isArray(TIMESTAMP_TSA_PRESETS)).toBe(true);
+    expect(TIMESTAMP_TSA_PRESETS.length).toBeGreaterThan(0);
+  });
+
+  it('should contain only objects with label and url strings', () => {
+    for (const preset of TIMESTAMP_TSA_PRESETS) {
+      expect(typeof preset.label).toBe('string');
+      expect(preset.label.length).toBeGreaterThan(0);
+      expect(typeof preset.url).toBe('string');
+      expect(preset.url.length).toBeGreaterThan(0);
+    }
+  });
+
+  it('should have unique labels', () => {
+    const labels = TIMESTAMP_TSA_PRESETS.map((p) => p.label);
+    expect(new Set(labels).size).toBe(labels.length);
+  });
+
+  it('should have unique URLs', () => {
+    const urls = TIMESTAMP_TSA_PRESETS.map((p) => p.url);
+    expect(new Set(urls).size).toBe(urls.length);
+  });
+
+  it('should have valid URL formats', () => {
+    for (const preset of TIMESTAMP_TSA_PRESETS) {
+      expect(() => new URL(preset.url)).not.toThrow();
+    }
+  });
+
+  it('should include well-known TSA providers', () => {
+    const labels = TIMESTAMP_TSA_PRESETS.map((p) => p.label);
+    expect(labels).toContain('DigiCert');
+    expect(labels).toContain('Sectigo');
+  });
+
+  it('should satisfy the TimestampTsaPreset interface', () => {
+    const preset: TimestampTsaPreset = TIMESTAMP_TSA_PRESETS[0];
+    expect(preset).toHaveProperty('label');
+    expect(preset).toHaveProperty('url');
+  });
+});