fix(dashboard): persist the currency & timezone widgets so an upgrade keeps them (#1311)

The currency and timezone widgets stored their state only in browser localStorage, so a (docker) upgrade that clears site storage reset them to defaults — unlike every other preference, which is saved server-side. Persist them through the per-user settings store (no schema change; the settings table takes arbitrary keys) and migrate any existing localStorage values on first load so users keep their picks. dashboard_timezones is left unset by default so the widget can tell 'never chosen' from an explicitly emptied list.

client/src/types.ts

@@ -124,6 +124,10 @@ export interface Settings {
   maplibre_style?: string
   mapbox_3d_enabled?: boolean
   mapbox_quality_mode?: boolean
+  // Dashboard widget prefs — persisted server-side so a (docker) upgrade keeps them (#1311).
+  dashboard_fx_from?: string
+  dashboard_fx_to?: string
+  dashboard_timezones?: string[]
 }
 
 export interface AssignmentsMap {