fix(admin-ui): Plex library toggle inversion + Purge-discovery confirm (+ cleanups)

Bugs:
- ConfigPanel (CB1): unchecking a Plex library from the "all" (empty) state made
  that library the ONLY selected one instead of all-except-it, because the toggle
  ADDED the key to an empty list. Expand "all" to the explicit section set before
  toggling. E2E-verified: uncheck from all → all-except-one.
- Scheduler (SB1): "Purge discovery" bulk-deleted search videos/videos/channels
  immediately with no confirm (every AdminUsers destructive action uses useConfirm).
  Wrapped in a danger useConfirm dialog (+ trilingual purgeConfirmTitle key).
  E2E-verified: dialog appears, Cancel aborts.

Cleanups (behavior-neutral):
- Register LS.configTab; ConfigPanel uses it instead of the bare "siftlode.configTab".
- ConfigPanel + SettingsPanel import SaveState from ui/DraftSaveBar instead of
  redeclaring the union (PrefsSaveState is now an alias).

tsc green, re-review clean, localdev boots.

frontend/src/components/ConfigPanel.tsx

@@ -7,7 +7,8 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import Tabs, { usePersistedTab } from "./Tabs";
 import { Switch } from "./ui/form";
-import { DraftSaveBar } from "./ui/DraftSaveBar";
+import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
+import { LS } from "../lib/storage";
 
 // Admin Configuration page: edit DB-overridable operational settings (registry-driven from the
 // backend — adding a key there makes it appear here automatically). Edits are drafted and
@@ -15,7 +16,6 @@ import { DraftSaveBar } from "./ui/DraftSaveBar";
 // the server until Save. Group order is fixed where known so logically related settings (e.g.
 // Email/SMTP) stay together.
 const GROUP_ORDER = ["access", "email", "youtube", "google", "quota", "backfill", "shorts", "batch", "downloads", "plex"];
-type SaveState = "idle" | "saving" | "saved" | "error";
 
 export default function ConfigPanel() {
   const { t } = useTranslation();
@@ -40,7 +40,7 @@ export default function ConfigPanel() {
   const [saveState, setSaveState] = useState<SaveState>("idle");
   // One tab per config group (persisted). Called before the loading guard so hook order is
   // stable; the active id is clamped to a real group at render time once data has loaded.
-  const [tab, setTab] = usePersistedTab("siftlode.configTab");
+  const [tab, setTab] = usePersistedTab(LS.configTab);
 
   // Re-seed the draft whenever the server data changes (initial load + after a save/reset).
   useEffect(() => {
@@ -110,9 +110,14 @@ export default function ConfigPanel() {
   });
   const plexLibs = (draft["plex_libraries"] ?? "").split(",").map((s) => s.trim()).filter(Boolean);
   const togglePlexLib = (key: string) => {
-    const next = plexLibs.includes(key)
-      ? plexLibs.filter((k) => k !== key)
-      : [...plexLibs, key];
+    // An empty list means "all libraries" — every box renders checked. Unchecking one from that
+    // state must select all-except-this, so expand "all" to the explicit section set first;
+    // otherwise the toggle would ADD the key and leave it as the ONLY selected library.
+    const allKeys = plexResult?.sections?.map((s) => s.key) ?? [];
+    const current = plexLibs.length === 0 ? allKeys : plexLibs;
+    const next = current.includes(key)
+      ? current.filter((k) => k !== key)
+      : [...current, key];
     setDraft((d) => ({ ...d, plex_libraries: next.join(",") }));
   };