feat: Implement basic general folder support (#2051)

- Related: #1748

tests/ha/side-load-ha-elements.test.ts

@@ -0,0 +1,62 @@
+import { LitElement } from 'lit';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { sideLoadHomeAssistantElements } from '../../src/ha/side-load-ha-elements';
+
+describe('sideLoadHomeAssistantElements', () => {
+  beforeEach(() => {
+    vi.stubGlobal('customElements', {
+      get: vi.fn(),
+      whenDefined: vi.fn(),
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('returns true if all elements already registered', async () => {
+    vi.mocked(customElements.get).mockResolvedValue(LitElement);
+    expect(await sideLoadHomeAssistantElements()).toBe(true);
+  });
+
+  it('returns false when the picture glance card cannot be found', async () => {
+    vi.mocked(customElements.get).mockReturnValue(undefined);
+
+    const createCardElement = vi.fn();
+    vi.stubGlobal('window', {
+      loadCardHelpers: vi.fn().mockReturnValue({
+        createCardElement,
+      }),
+    });
+
+    expect(await sideLoadHomeAssistantElements()).toBe(false);
+  });
+
+  it('returns true when elements are loaded', async () => {
+    vi.mocked(customElements.get).mockImplementation((name: string) => {
+      if (name === 'hui-picture-glance-card') {
+        const result = LitElement;
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        (result as any).getConfigElement = vi.fn();
+        return LitElement;
+      }
+      return undefined;
+    });
+
+    const createCardElement = vi.fn();
+    vi.stubGlobal('window', {
+      loadCardHelpers: vi.fn().mockReturnValue({
+        createCardElement,
+      }),
+    });
+
+    expect(await sideLoadHomeAssistantElements()).toBe(true);
+
+    expect(customElements.whenDefined).toHaveBeenCalledWith('hui-picture-glance-card');
+    expect(createCardElement).toHaveBeenCalledWith({
+      type: 'picture-glance',
+      entities: [],
+      camera_image: 'dummy-to-load-editor-components',
+    });
+  });
+});