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/Scheduler.tsx

@@ -18,6 +18,7 @@ import {
 } from "lucide-react";
 import { api, type SchedulerJob, type SchedulerStatus } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
+import { useConfirm } from "./ConfirmProvider";
 import { relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
@@ -309,6 +310,7 @@ function Stat({
 export default function Scheduler() {
   const { t } = useTranslation();
   const qc = useQueryClient();
+  const confirm = useConfirm();
   // Poll faster while any job is running so live progress is smooth, then ease back when
   // idle. Derived from the freshest data by react-query itself (see useLiveQuery).
   const q = useLiveQuery<SchedulerStatus>(["scheduler"], api.schedulerStatus, {
@@ -411,7 +413,15 @@ export default function Scheduler() {
         </Tooltip>
         <Tooltip hint={t("scheduler.purgeDiscoveryHint")}>
           <button
-            onClick={() => purgeMut.mutate()}
+            onClick={async () => {
+              const ok = await confirm({
+                title: t("scheduler.purgeConfirmTitle"),
+                message: t("scheduler.purgeDiscoveryHint"),
+                confirmLabel: t("scheduler.purgeDiscovery"),
+                danger: true,
+              });
+              if (ok) purgeMut.mutate();
+            }}
             disabled={purgeMut.isPending}
             className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
           >