Replace markdown editor with native Tiptap editor and add Home navigation

- Migrate renderer from md-editor/md-format to a Tiptap-based native editor
  (native-doc pipeline: native-doc, markdown-to-native, native-to-markdown,
  native-assembly, migration, backup)
- Use pointer-event-based chapter reordering with grab/grabbing cursors to
  avoid the Wayland prohibited-drop cursor while preserving reordering
- Add a persistent Home button that closes the open book and returns to the
  Folio library/welcome screen
- Update app icons, package deps, theme styles, and tests

tests/.folio-fullbook-test-entry.ts

@@ -0,0 +1,14 @@
+// Headless test entry for the native Full Book assembly + rendering.
+// Run with: node tests/run-fullbook-test.mjs
+export {
+  buildFullBookDoc,
+  nativeNodeText,
+  chapterHasOwnHeading,
+} from "../src/renderer/native-assembly.ts";
+export {
+  createTiptapEditor,
+  setEditorDoc,
+  getEditorDoc,
+  ChapterBreak,
+} from "../src/renderer/tiptap-editor.ts";
+export { CURRENT_DOC_SCHEMA_VERSION } from "../src/main/native-doc.ts";