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/data/seed.ts

@@ -1,4 +1,7 @@
 /**
+ * Document templates: `createBlankDocument()` (Datei › Neu, and what a first
+ * start opens) and `createSeedDocument()` (Datei › Muster).
+ *
  * The pre-filled document ("roter Faden" of the study), in the v2 canvas model:
  * one sheet ("Analyse") carrying three table objects and a chart object, plus a
  * second empty sheet to try things out.
@@ -144,3 +147,21 @@ export function createSeedDocument(): DocumentState {
     macros: [],
   };
 }
+
+/** An empty document: one sheet with one empty table, ready to type into. */
+export function createBlankDocument(): DocumentState {
+  const table = makeTable('Tabelle 1', 40, 40, 12, 6, []);
+  const sheet = { id: nextId('sheet'), name: 'Blatt 1', objects: [table] };
+  return {
+    version: 2,
+    sheets: [sheet],
+    activeSheetId: sheet.id,
+    activeObjectId: table.id,
+    selection: { tableId: table.id, anchor: { col: 0, row: 0 }, focus: { col: 0, row: 0 } },
+    locale: 'de-DE',
+    csvDelimiter: ',',
+    customFunctions: [],
+    namedRanges: [],
+    macros: [],
+  };
+}