Start with an empty document; offer Datei › Neu and Datei › Muster

A first start (nothing in localStorage) now opens a blank document -- one
sheet "Blatt 1" with an empty "Tabelle 1" -- instead of the pre-filled
sample. The file menu gets "Neu" (blank document) and "Muster ·
Umsatzanalyse" (the former seed), both behind a confirm and both starting
a fresh undo history. The toolbar's "Zurücksetzen" button is gone; its job
is now Datei › Muster.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/store/__tests__/document.spec.ts

@@ -120,6 +120,19 @@ describe('seed document acceptance (canvas model)', () => {
     expect(val(s, 'Übersicht', 'B2')).toBe(5970);
   });
 
+  it('newDocument replaces everything with one empty sheet + table and a fresh history', () => {
+    const s = fresh();
+    setCell(s, 'Umsatz', 'B2', '99999');
+    s.newDocument();
+    expect(s.doc.sheets).toHaveLength(1);
+    expect(s.doc.sheets[0].name).toBe('Blatt 1');
+    const [table] = s.doc.sheets[0].objects;
+    expect(table.kind === 'table' && table.name).toBe('Tabelle 1');
+    expect(table.kind === 'table' && Object.keys(table.cells)).toEqual([]);
+    expect(s.doc.namedRanges).toEqual([]);
+    expect(s.canUndo.value).toBe(false);
+  });
+
   it('inserting a new table and referencing it across sheets works', () => {
     const s = fresh();
     const sheet2 = s.doc.sheets[1].id;