Fix blank editor on book open and add home delete + auto-folder new book

- Auto-select the first chapter when a book opens so the editor is no
  longer left on the placeholder (was the main `all I see is the title`
  symptom).
- Stop markdownToNative from emitting text nodes with empty content and
  defensively strip them in setEditorDoc; ProseMirror rejects such nodes,
  which made editor.commands.setContent silently empty the whole editor.
- Home screen: delete a book (sent to trash, recoverable) and create a new
  book in a folder named after it under ~/Documents.
- Add self-test coverage for new book + delete.

tests/selftest-scenario.js

@@ -92,10 +92,10 @@
 
         // Manual save: typing marks the chapter Unsaved; the Save button (or
         // Ctrl/Cmd+S) is required to persist. No auto-save should occur.
-        const ed = window.__folioEditor;
-        if (ed) {
-          ed.commands.focus("end");
-          ed.commands.insertContent(" manual-save-check");
+        const edSave = window.__folioEditor;
+        if (edSave) {
+          edSave.commands.focus("end");
+          edSave.commands.insertContent(" manual-save-check");
           await sleep(150);
           const unsaved = document.getElementById("saveState");
           step("afterTypeUnsaved", unsaved ? unsaved.textContent : "n/a");
@@ -170,6 +170,18 @@
         }
         step("exports", ex);
 
+        // New book auto-creates a folder inside Documents; then delete it.
+        const nb = await api.newBookNamed("Folio Selftest Temp");
+        step("newBookNamed", {
+          ok: !!(nb && nb.path && nb.title),
+          path: nb && nb.path,
+          title: nb && nb.title,
+        });
+        if (nb && nb.path) {
+          const del = await api.deleteBook(nb.path);
+          step("deleteBook", del && del.ok ? "ok" : del && del.error);
+        }
+
         out.ok = true;
       } catch (e) {
         out.error = String((e && e.stack) || e);