Fix timezone consistency: apply server timezone everywhere in the app

- Extract getServerTimezone() helper in timeUtils.js; apply it in
  formatChartTime and formatLocalTimeOnly (previously ignored TZ)
- Status.js: remove local formatLocalTime (was bare toLocaleString),
  import the shared util that reads server timezone
- SettingsWizard: immediately update localStorage when timezone is saved
  so new value takes effect without waiting for cache to expire

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/Status.js

@@ -1,5 +1,6 @@
 import React, { useEffect, useState } from 'react';
 import axios from 'axios';
+import { formatLocalTime } from './utils/timeUtils';
 import './Status.css';
 
 function Status() {
@@ -31,14 +32,6 @@ function Status() {
     }
   };
 
-  const formatLocalTime = (timestamp) => {
-    try {
-      return new Date(timestamp).toLocaleString();
-    } catch {
-      return timestamp;
-    }
-  };
-
   const formatDuration = (ms) => {
     const seconds = Math.floor(ms / 1000);
     const minutes = Math.floor(seconds / 60);