fix(18): enable first-run timezone save when not explicitly set (WR-01)

- Derive isExplicit from timezoneQuery.data?.isExplicitlySet
- Apply the input===stored no-op guard only when isExplicit is true
- Keep pending and empty-input guards unconditional
- Add unit tests (AdminPage.timezone.test.ts) verifying first-run Save is
  enabled when isExplicitlySet:false and input matches stored fallback value

apps/pwa/src/routes/AdminPage.tsx

@@ -111,11 +111,21 @@ export function AdminPage() {
   const storedTimezone = timezoneQuery.data?.timezone ?? '';
   const effectiveTimezoneInput = timezoneInput ?? storedTimezone;
 
-  // Save is disabled when pending, or when input matches what's stored
+  // WR-01: derive isExplicit so we only apply the no-op guard when the timezone
+  // has ALREADY been explicitly saved. On first run (isExplicitlySet: false) the
+  // admin must be able to confirm/save the displayed system-default — even if the
+  // input value already matches the fallback string. Keep pending and empty-input
+  // guards unconditional.
+  const isExplicit = timezoneQuery.data?.isExplicitlySet ?? false;
+
+  // Save is disabled when:
+  //   - mutation is in-flight (pending), OR
+  //   - input is empty, OR
+  //   - the timezone IS already explicitly set AND the input is unchanged (no-op)
   const timezoneSaveDisabled =
     timezoneMutation.isPending ||
     effectiveTimezoneInput === '' ||
-    effectiveTimezoneInput === storedTimezone;
+    (isExplicit && effectiveTimezoneInput === storedTimezone);
 
   // IANA zones list for the datalist (Intl.supportedValuesOf may not be present in all runtimes)
   const ianaZones: string[] =