Add blank workbook template, example rules file, and Create blank workbook button

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

app.py

@@ -155,6 +155,13 @@ class H(BaseHTTPRequestHandler):
                     keep = L.BACKUPS / f"before_undo_{dt.datetime.now():%Y%m%d_%H%M%S}.xlsx"
                     shutil.copy2(L.WORKBOOK, keep); shutil.copy2(b[-1], L.WORKBOOK)
                 return self._send(200, {"message": f"Restored the workbook from {b[-1].name}."})
+            if u.path == "/api/create_blank":
+                tpl = HERE / "template" / "Bills_Tracker.template.xlsx"
+                with LOCK:
+                    if L.WORKBOOK.exists(): return self._send(400, {"error": "Bills_Tracker.xlsx already exists; it won't be overwritten."})
+                    if not tpl.exists(): return self._send(400, {"error": "Template not found in the app."})
+                    DATA.mkdir(parents=True, exist_ok=True); shutil.copy2(tpl, L.WORKBOOK)
+                return self._send(200, {"message": "Created a blank Bills_Tracker.xlsx. Now drop your statements in."})
             if u.path == "/api/open":
                 if sys.platform != "darwin": return self._send(200, {"message": "Open Bills_Tracker.xlsx from the data folder on the host."})
                 subprocess.run(["open", str(L.WORKBOOK)], check=False)