Add guide metadata, redesign PDF cover, paginate steps per-page, and run exports in a helper process

Lets users record author/co-authors/organization for a guide via a new
"Guide information…" dialog; this metadata renders below the title on
the PDF cover (title now sits above the accent rule). PDF export also
paginates so each step fits its own page where possible, keeps a
step's title/image/lead-in together, and forces the next step onto a
fresh page after an oversized step overflows. Exports now run in a
forked helper process so large guides no longer freeze the UI.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

tests/unit/store.test.js

@@ -213,3 +213,27 @@ test('guide ids are validated against path traversal', (t) => {
   assert.throws(() => store.guideDir('a/b'));
   assert.throws(() => store.stepDir('ok', '..'));
 });
+
+test('guide metadata: optional fields default to empty strings and round-trip via save/load', (t) => {
+  const root = makeTmpDir('metadata');
+  t.after(() => rmrf(root));
+  const store = new GuideStore(root);
+  const guide = store.createGuide({ title: 'Metadata test' });
+  assert.deepEqual(guide.metadata, { author: '', coAuthors: '', organization: '' });
+
+  guide.metadata = { author: 'Jane Doe', coAuthors: 'Alex Lee, Sam Patel', organization: 'Acme Corp' };
+  store.saveGuide(guide);
+
+  const fresh = new GuideStore(root);
+  const loaded = fresh.getGuide(guide.guideId);
+  assert.deepEqual(loaded.metadata, { author: 'Jane Doe', coAuthors: 'Alex Lee, Sam Patel', organization: 'Acme Corp' });
+
+  // A guide saved before metadata existed normalizes to the same defaults.
+  const legacy = fresh.getGuide(guide.guideId);
+  delete legacy.metadata;
+  store.saveGuide(legacy);
+  assert.deepEqual(fresh.getGuide(guide.guideId).metadata, { author: '', coAuthors: '', organization: '' });
+
+  loaded.metadata = 'not an object';
+  assert.throws(() => store.saveGuide(loaded), /metadata must be an object/);
+});