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/lib/storage.ts

@@ -19,6 +19,7 @@ export const LS = {
   channelsTable: "siftlode.channelsTable",
   channelDiscoveryTable: "siftlode.channelDiscoveryTable",
   settingsTab: "siftlode.settingsTab",
+  configTab: "siftlode.configTab",
   statsTab: "siftlode.statsTab",
   adminUsersTab: "siftlode.adminUsersTab",
   navCollapsed: "siftlode.navCollapsed",