Add new categories/envelopes from the Budget page and the loader's categorize card

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

app.py

@@ -172,6 +172,12 @@ class H(BaseHTTPRequestHandler):
                 req = json.loads(body or b"{}")
                 with LOCK: out = L.set_budget(req["month"], req["budgets"])
                 return self._send(200, {"message": f"Saved the {req['month']} budget ({out['rows']} envelopes). Backup: {out['backup']}."})
+            if u.path == "/api/add_category":
+                try:
+                    with LOCK: out = L.add_category(json.loads(body or b"{}").get("name"))
+                except ValueError as e: return self._send(400, {"error": str(e)})
+                note = " It will show on the web pages; the Excel summary tabs only have 20 category slots." if out["beyond_excel"] else ""
+                return self._send(200, {"message": f"Added category '{out['name']}'.{note} Backup: {out['backup']}.", "name": out["name"]})
             if u.path == "/api/set_categories":
                 with LOCK: out = L.set_categories(json.loads(body or b"[]"))
                 return self._send(200, {"message": f"Updated {out['updated']} payee(s). Backup: {out['backup']}."})