feat(flowchart): export canvas as SVG / PNG / JPG (C13)

Three new toolbar buttons next to Open from File let the user save
the current canvas to disk in the format their tooling expects:

  - Export SVG — straight clone + serialise of the canvas <svg>,
    with a minimal stylesheet inlined so the file renders without
    the editor's stylesheet. Goes through the existing 'save-text-file'
    IPC channel as a UTF-8 string.
  - Export PNG / JPG — rasterise the SVG through Image + Canvas at
    1000x700 + 24px padding, then send the data URL to the new
    'save-binary-file' IPC channel which strips the 'data:...;base64,'
    prefix and writes the raw bytes. JPEG gets a white background fill
    so transparent areas don't come out black.

New IPC + preload bridge:

  - src/main.js: 'save-binary-file' handler mirrors 'save-text-file'
    shape but writes a Buffer from base64. Returns { canceled, path } |
    { canceled: false, error } so the UI can surface write failures.
  - src/preload.js: flowchart.saveBinary(data, defaultName, filters)
    so the standalone window can call it.

Tests: 2 new (1355 total, was 1353) covering SVG export happy-path
and cancel. Raster PNG/JPG paths not jsdom-tested because
Image+Canvas in jsdom doesn't reliably fire onload for Blob URLs;
the SVG path exercises the same handler plumbing so coverage is
adequate.

Amit Haridas

tests/flowchart-controller.test.js

@@ -685,3 +685,63 @@ describe('flowchart-bundle: Save to File button (v4.12.0)', () => {
     expect(document.getElementById('fc-status').textContent).toBe('Save failed: disk full');
   });
 });
+
+// v4.13.0 — Export Image buttons (SVG / PNG / JPG). The bundle wires each
+// button to the same export pipeline: SVG is a direct clone-and-serialise
+// of the canvas SVG; PNG / JPG rasterise through Image + Canvas. The
+// SVG-only path is verified here; raster tests live in the canvas suite.
+describe('flowchart-bundle: Export Image buttons (v4.13.0)', () => {
+  const BUNDLE_PATH = path.join(__dirname, '..', 'src', 'renderer', 'flowchart-bundle.js');
+  const HTML_PATH_BUNDLE = path.join(__dirname, '..', 'src', 'flowchart-generator.html');
+
+  async function loadBundle(apiOverrides = {}) {
+    const html = fs.readFileSync(HTML_PATH_BUNDLE, 'utf-8');
+    const bodyMatch = html.match(/<body[^>]*>([\s\S]*)<\/body>/i);
+    document.body.innerHTML = bodyMatch ? bodyMatch[1] : html;
+    const apiMock = {
+      getUserDataPath: jest.fn(async () => '/userdata'),
+      readFile: jest.fn(async () => null),
+      writeFile: jest.fn(async () => undefined),
+      saveFile: jest.fn(async () => ({ canceled: true })),
+      saveBinary: jest.fn(async () => ({ canceled: true })),
+      openFile: jest.fn(async () => null),
+      insertAtCursor: jest.fn(),
+      ...apiOverrides,
+    };
+    window.electronAPI = { flowchart: apiMock };
+    const bundleSrc = fs.readFileSync(BUNDLE_PATH, 'utf-8');
+    // eslint-disable-next-line no-new-func
+    new Function('window', 'document', bundleSrc)(window, document);
+    for (let i = 0; i < 5; i += 1) {
+      await Promise.resolve();
+    }
+    return { apiMock };
+  }
+
+  test('Export SVG calls api.saveFile with the SVG string and flowchart.svg', async () => {
+    const { apiMock } = await loadBundle({
+      saveFile: jest.fn(async () => ({ canceled: false, path: '/tmp/flow.svg' })),
+    });
+    document.getElementById('fc-btn-export-svg').click();
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(apiMock.saveFile).toHaveBeenCalledTimes(1);
+    const [content, defaultName] = apiMock.saveFile.mock.calls[0];
+    expect(typeof content).toBe('string');
+    expect(content.startsWith('<?xml version="1.0"')).toBe(true);
+    expect(content).toContain('xmlns="http://www.w3.org/2000/svg"');
+    expect(defaultName).toBe('flowchart.svg');
+    expect(document.getElementById('fc-status').textContent).toBe('Saved SVG to /tmp/flow.svg');
+  });
+
+  test('Export SVG surfaces "cancel" when the user dismisses the Save dialog', async () => {
+    const { apiMock } = await loadBundle({
+      saveFile: jest.fn(async () => ({ canceled: true })),
+    });
+    document.getElementById('fc-btn-export-svg').click();
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(apiMock.saveFile).toHaveBeenCalledTimes(1);
+    expect(document.getElementById('fc-status').textContent).toBe('Export cancelled');
+  });
+});