feat: Implement basic general folder support (#2051)

- Related: #1748

tests/ha/is-ha-relative-url.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest';
+import { isHARelativeURL } from '../../src/ha/is-ha-relative-url';
+
+describe('isHARelativeURL', () => {
+  it('returns true for a simple relative URL', () => {
+    expect(isHARelativeURL('/image.jpg')).toBe(true);
+  });
+
+  it('returns true for root path', () => {
+    expect(isHARelativeURL('/')).toBe(true);
+  });
+
+  it('returns false for undefined', () => {
+    expect(isHARelativeURL()).toBe(false);
+  });
+
+  it('returns false for empty string', () => {
+    expect(isHARelativeURL('')).toBe(false);
+  });
+
+  it('returns false for absolute URL', () => {
+    expect(isHARelativeURL('https://card.camera/image.jpg')).toBe(false);
+  });
+});