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/store/settingsStore.ts

@@ -41,6 +41,10 @@ export const useSettingsStore = create<SettingsState>((set, get) => ({
     maplibre_style: '',
     mapbox_3d_enabled: true,
     mapbox_quality_mode: false,
+    dashboard_fx_from: 'EUR',
+    dashboard_fx_to: 'USD',
+    // dashboard_timezones is intentionally left unset so the widget can tell "never
+    // chosen" (fall back to home + defaults) from an explicitly emptied list.
   },
   isLoaded: false,