[idumlupinar]

Closes #7579

Summary

The Site Time Zone dropdown (Persona Bar → Settings → Site Settings → Site Info) lists every Windows time zone (144 on a current Windows Server), ordered by UTC offset. Finding a zone meant scrolling through the whole list.

This PR turns on the shared Dropdown component's existing withSearch option for that field, the same way the Language editor already does. The dropdown now shows a search box at the top of the list, focused when it opens, that filters as you type.

• Matching is case-insensitive and finds the text anywhere in the label or the time zone ID. You can search by city or region (istanbul), offset (+05:30), or Windows time zone ID (turkey → (UTC+03:00) Istanbul).
• Adds two localized strings to SiteSettings.resx: the search placeholder and the "no results" message.
• No changes to the shared component, the API, or how the setting is saved.

Release Note

The Site Time Zone dropdown in Site Settings now has a search box, so administrators can find a time zone by city, region or UTC offset instead of scrolling through the full list.

Testing Steps

1. Persona Bar → Settings → Site Settings → Site Info.
2. Open the Site Time Zone dropdown. A search box appears at the top with the cursor in it.
3. Type istanbul. Only (UTC+03:00) Istanbul is listed.
4. Type pacific. Pacific zones are listed, e.g. (UTC-08:00) Pacific Time (US & Canada).
5. Type +05:30. The two UTC+05:30 zones are listed.
6. Type zzz. "No matching time zones found." is shown.