✨ feat(form): let callers preselect a fallback timezone

- ✨ an offline install has no geo guess, so the timezone list opened on Africa/Abidjan; a caller-supplied zone now stands in for the missing guess
- ✅ a successful geo guess still outranks the fallback

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SdXqTwm63rRMwotZ1xqLC8

install/provisioning/setup-form.sh

@@ -167,10 +167,13 @@ omarchy_prompt_hostname() {
 }
 
 # A fresh machine often hasn't joined a network yet, so the geo guess fails
-# often; guard it or a `set -e` caller dies before the filter fallback.
+# often; guard it or a `set -e` caller dies before the filter fallback. Callers
+# may pass the zone to preselect when there is no guess (a region's default),
+# so an offline install does not open on the top of an alphabetical list.
 omarchy_prompt_timezone() {
   local guess status
   guess=$(tzupdate -p 2>/dev/null) || guess=""
+  [[ -n $guess ]] || guess="${1:-}"
 
   if [[ -n $guess ]]; then
     timezone=$(timedatectl list-timezones | gum choose --height 10 --selected "$guess" --header "Timezone") && status=0 || status=$?