✨ 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

test/shell.d/setup-form-test.sh

@@ -59,7 +59,7 @@ fi
 
 trap 'if [[ ${FUNCNAME[0]:-} == "$PROMPT_FN" ]]; then printf "returned\n" >>"$MARKER"; fi' RETURN
 
-"$PROMPT_FN"
+"$PROMPT_FN" ${PROMPT_ARG:+"$PROMPT_ARG"}
 
 printf 'keyboard=%s\n' "${keyboard:-}"
 printf 'keyboard_label=%s\n' "${keyboard_label:-}"
@@ -225,6 +225,19 @@ assert_status 0 "timezone prompt survives a failed geo guess"
 [[ $(head -n 1 "$GUM_ARGS") == filter* ]] || fail "timezone prompt filters when there is no geo guess"
 pass "timezone prompt falls back to filtering when the geo guess fails"
 
+# A region's default stands in for the missing guess, so an offline China
+# install opens on Asia/Shanghai instead of the top of the list
+PROMPT_ARG=Asia/Shanghai run_prompt omarchy_prompt_timezone "0:Asia/Shanghai"
+assert_status 0 "timezone prompt accepts the caller's fallback"
+[[ $(field timezone) == "Asia/Shanghai" ]] || fail "timezone prompt keeps the preselected fallback"
+grep -qF -- '--selected Asia/Shanghai' "$GUM_ARGS" || fail "timezone prompt preselects the caller's fallback"
+[[ $(head -n 1 "$GUM_ARGS") == choose* ]] || fail "timezone prompt chooses from the list when it has a fallback"
+pass "timezone prompt preselects the caller's fallback when the geo guess fails"
+
+TZ_GUESS=Europe/Copenhagen PROMPT_ARG=Asia/Shanghai run_prompt omarchy_prompt_timezone "0:Europe/Copenhagen"
+grep -qF -- '--selected Europe/Copenhagen' "$GUM_ARGS" || fail "a geo guess outranks the caller's fallback"
+pass "timezone prompt prefers the geo guess over the caller's fallback"
+
 run_prompt omarchy_prompt_timezone "0:"
 assert_status 0 "timezone prompt accepts an empty selection"
 [[ $(field timezone) == "UTC" ]] || fail "timezone prompt falls back to UTC"