Make the admin timezone an explicit manual picker (task ade35bd5)

Replace the free-text Timezone box with a select of curated IANA zones,
an "API host local time" choice that clears the zone, and an "Other IANA
zone…" choice that opens a text entry for any other zone. A saved zone
outside the list opens in that entry on load. Helper text states this is
the application timezone and the source of local time for the panel,
scheduler and admin app. Saving still patches only changed fields, so a
timezone change sends exactly {"timezone": ...}; nothing is written on
load and the browser zone is never read.

API tests add invalid-zone 422 cases, a non-admin timezone PATCH refusal,
and a lone timezone PATCH sticking on a fresh database.

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

admin/src/settings/SettingsScreen.tsx

@@ -6,7 +6,8 @@
  *
  * Preferences: the household settings (planning horizon, day rollover,
  * timezone, notifications). Save patches only the changed fields. The
- * timezone is only ever set by the admin saving it here.
+ * timezone — the application's source of local time — is only ever set by
+ * the admin picking or typing a zone here and saving it.
  *
  * One write at a time across both sections.
  */
@@ -256,7 +257,7 @@ const HH_MM = /^([01]\d|2[0-3]):[0-5]\d$/;
 /** "UTC" or an IANA-looking "Area/Location" (the API checks it resolves). */
 const IANA_ZONE = /^(UTC|[A-Za-z]+(\/[A-Za-z0-9_+-]+)+)$/;
 
-/** The Timezone field's suggestions; any other IANA zone can be typed. */
+/** The Timezone picker's zones; any other IANA zone can be typed via "Other". */
 export const COMMON_TIMEZONES = [
   "UTC",
   "America/New_York",
@@ -290,6 +291,9 @@ export const COMMON_TIMEZONES = [
   "Pacific/Auckland",
 ];
 
+/** The picker's "Other IANA zone…" option; never sent to the API. */
+const OTHER_ZONE = "__other__";
+
 /** The form's working copy: the number is text so it can be mid-edit. */
 type PreferencesDraft = Omit<HouseholdSettings, "horizon_days"> & { horizon_days: string };
 
@@ -348,6 +352,10 @@ function PreferencesForm({
 }) {
   const [draft, setDraft] = useState<PreferencesDraft>(() => preferencesDraft(settings));
   const update = (patch: Partial<PreferencesDraft>) => setDraft((d) => ({ ...d, ...patch }));
+  // A saved zone outside the curated list opens in the free-text entry.
+  const [otherZone, setOtherZone] = useState(
+    () => settings.timezone !== "" && !COMMON_TIMEZONES.includes(settings.timezone),
+  );
   const disabled = saving || locked;
 
   function submit(event: FormEvent) {
@@ -400,25 +408,49 @@ function PreferencesForm({
         />
       </label>
       {timeInput("day_rollover_time", "Day rollover")}
-      <label className="settings-field">
-        <span className="settings-label">Timezone</span>
-        <input
+      <div className="settings-field">
+        <label className="settings-label" htmlFor="settings-timezone">
+          Timezone
+        </label>
+        <select
+          id="settings-timezone"
           className="settings-input"
-          type="text"
-          list="settings-timezones"
-          placeholder="Host local time"
-          autoComplete="off"
-          spellCheck={false}
-          value={draft.timezone}
+          aria-describedby="settings-timezone-help"
+          value={otherZone ? OTHER_ZONE : draft.timezone}
           disabled={disabled}
-          onChange={(event) => update({ timezone: event.target.value })}
-        />
-      </label>
-      <datalist id="settings-timezones">
-        {COMMON_TIMEZONES.map((zone) => (
-          <option key={zone} value={zone} />
-        ))}
-      </datalist>
+          onChange={(event) => {
+            const choice = event.target.value;
+            setOtherZone(choice === OTHER_ZONE);
+            if (choice !== OTHER_ZONE) update({ timezone: choice });
+          }}
+        >
+          <option value="">API host local time (no zone set)</option>
+          {COMMON_TIMEZONES.map((zone) => (
+            <option key={zone} value={zone}>
+              {zone}
+            </option>
+          ))}
+          <option value={OTHER_ZONE}>Other IANA zone…</option>
+        </select>
+        {otherZone ? (
+          <input
+            className="settings-input"
+            type="text"
+            aria-label="Other timezone"
+            placeholder="Area/Location, e.g. America/Boise"
+            autoComplete="off"
+            spellCheck={false}
+            value={draft.timezone}
+            disabled={disabled}
+            onChange={(event) => update({ timezone: event.target.value })}
+          />
+        ) : null}
+        <p className="settings-hint" id="settings-timezone-help">
+          The application timezone: the source of local time for the panel, the scheduler and this
+          admin app. It changes only when you save it here; with no zone set, the API host&apos;s
+          local time is used.
+        </p>
+      </div>
       <label className="settings-field">
         <span className="settings-label">Notify target</span>
         <input