fix(ui): centralize default layout width and update default setting

internal/site/src/lib/stores.ts

@@ -3,6 +3,9 @@ import type { AlertMap, ChartTimes, SystemRecord, UserSettings } from "@/types"
 import { pb } from "./api"
 import { Unit } from "./enums"
 
+/** Default layout width. Used as fallback when user setting is unset. */
+export const defaultLayoutWidth = 1580
+
 /** Store if user is authenticated */
 export const $authenticated = atom(pb.authStore.isValid)