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/SettingsPanel.tsx

@@ -9,14 +9,14 @@ import Avatar from "./Avatar";
 import { notify, type NotifSettings } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import { Section, SettingRow, Switch } from "./ui/form";
-import { DraftSaveBar } from "./ui/DraftSaveBar";
+import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
 import { useConfirm } from "./ConfirmProvider";
 
 // The Settings page edits server-persisted preferences as a draft: changes apply locally
 // for instant preview but reach the server only on an explicit Save (or revert on Discard).
 // App owns the draft + baseline (so the leave-the-page guard can see it); this controller is
 // how the panel reads/writes it. See App.tsx.
-export type PrefsSaveState = "idle" | "saving" | "saved" | "error";
+export type PrefsSaveState = SaveState;
 export interface PrefsController {
   theme: ThemePrefs;
   setTheme: (t: ThemePrefs) => void;