Add the household settings screen

/settings/household edits the time zone (a searchable IANA list with each
zone's current UTC offset), dinner time, day-of reminder, rotation check,
and the plan draft, new suggestions and Google Chat Supply run day and
time, with plain-word copy for each and "Changes apply to everyone in the
household." Save PATCHes only the changed fields; the API's
invalid_settings problems show under their fields. The leave guard and
confirm sheet move out of the AI settings page so both screens share them.

Maestro task 48f8b8c3-7d5c-415d-9a61-980dc065bb90.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/settings/LeaveGuard.module.css

@@ -0,0 +1,38 @@
+/* The confirm sheet the settings screens share. */
+.sheet {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+  max-width: 560px;
+  margin: 0 auto;
+}
+
+.title {
+  margin: 0;
+  font-family: var(--font-heading);
+  font-size: 22px;
+  font-weight: 400;
+}
+
+.text {
+  margin: 0;
+  font-family: var(--font-body);
+  font-size: 14px;
+  line-height: 1.45;
+  color: var(--color-neutral-700);
+}
+
+.error {
+  margin: 0;
+  font-family: var(--font-body);
+  font-size: 13px;
+  font-weight: 700;
+  line-height: 1.4;
+  color: var(--color-accent-700);
+}
+
+.actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 10px;
+}