fix(ui): a page's sub-tab is remembered per browser tab, not forever

Settings, Configuration, Statistics and Users kept the last sub-tab in localStorage, so every
fresh tab opened Configuration on "Plex" as if that were its home. useTabPersistedState keeps it
in sessionStorage (account-scoped): a reload and a trip to another page keep it, a new tab starts
on the first sub-tab. The pre-selects (a Review link, the Google-link return) write the same store;
a value an older build left in localStorage is cleared on first read.

frontend/src/lib/storage.ts

@@ -42,6 +42,8 @@ export const LS = {
   channelDiscoveryCardSize: "siftlode.channelDiscoveryCardSize",
   channelsTable: "siftlode.channelsTable",
   channelDiscoveryTable: "siftlode.channelDiscoveryTable",
+  // A page's sub-tab: sessionStorage, per browser tab (useTabPersistedState), like adminUsersTab and
+  // statsTab below.
   settingsTab: "siftlode.settingsTab",
   configTab: "siftlode.configTab",
   auditTable: "siftlode.auditTable",
@@ -174,6 +176,61 @@ export function useAccountPersistedState(
   return [value, set];
 }
 
+// --- a page's sub-tab, per browser tab ---------------------------------------------------------
+
+// Which sub-tab a page was left on (Settings, Configuration, Statistics, Users) is kept for THIS browser
+// tab only — sessionStorage, account-scoped: it survives a reload and a trip to another page, and a new
+// tab opens the page on its first sub-tab (the owner, E31: a position remembered forever in
+// localStorage made Configuration open on "Plex" in every fresh tab, as if that were its home).
+
+/** The sub-tab stored for this browser tab, or null. */
+function getTabRaw(base: string): string | null {
+  const k = accountKey(base);
+  try {
+    return k ? sessionStorage.getItem(k) : null;
+  } catch {
+    return null;
+  }
+}
+
+/** Pre-select a page's sub-tab before navigating there (a "Review" link, the Google-link return). */
+export function setTabRaw(base: string, value: string): void {
+  const k = accountKey(base);
+  if (!k) return;
+  try {
+    sessionStorage.setItem(k, value);
+  } catch {
+    /* ignore */
+  }
+}
+
+/** `useAccountPersistedState` for a page's sub-tab, kept per browser tab (see above). */
+export function useTabPersistedState(
+  base: string,
+  fallback = "",
+): [string, (value: string) => void] {
+  const [value, setValue] = useState<string>(() => {
+    // Before E31 the position lived in localStorage; a value left there would never be read again.
+    const old = accountKey(base);
+    if (old) {
+      try {
+        localStorage.removeItem(old);
+      } catch {
+        /* ignore */
+      }
+    }
+    return getTabRaw(base) ?? fallback;
+  });
+  const set = useCallback(
+    (next: string) => {
+      setValue(next);
+      setTabRaw(base, next);
+    },
+    [base],
+  );
+  return [value, set];
+}
+
 /** A per-account persisted JSON OBJECT: `patch(partial)` merges + saves, unknown/missing keys fall
  * back to `defaults` (via readAccountMerged) so adding a field later is safe. Used for bundles of
  * related per-account settings (e.g. the Plex player prefs: volume, offsets, seek steps, sub style). */