refactor(fe): eliminate every explicit any; enforce no-explicit-any (R7 S3a·4)

Removes all 13 remaining `any`s and flips the ESLint rule from off to error so
new ones can't creep back:
- api.ts prefs/data JSON bags: Record<string, any> -> Record<string, unknown>,
  narrowed at consumers (App language guard; NotificationsPanel shapes the
  notification `data` per type behind its existing typeof guards)
- catch (e: any) -> catch (e) + `e instanceof HttpError` narrowing
  (SettingsPanel, SetupWizard, PlexPlayer)
- the YouTube IFrame API boundary: new lib/youtube.ts types (YTPlayer/
  YTPlayerEvent/YTNamespace + a Window augmentation) replace the 6 player anys;
  tsc surfaced the exact method surface to declare
- NotificationsPanel `t` props: any-options -> i18next TFunction

Gate green: typecheck (app+node+e2e), eslint 0 errors (rule now enforced),
prettier, 74 vitest.

frontend/src/components/NotificationsPanel.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useSyncExternalStore } from "react";
 import { useTranslation } from "react-i18next";
+import type { TFunction } from "i18next";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import {
@@ -223,7 +224,7 @@ function NotificationRow({
   onOpenScheduler,
 }: {
   n: AppNotification;
-  t: (k: string, o?: any) => string;
+  t: TFunction;
   onRead: () => void;
   onDismiss: () => void;
   onOpenScheduler: () => void;
@@ -239,15 +240,18 @@ function NotificationRow({
   let title = n.title;
   let body = n.body;
   if (isMaintenance) {
+    // Guarded above (typeof count === "number"); shape the JSON `data` bag for typed access.
+    const d = n.data as { count: number };
     title = t("inbox.maintenance.title");
-    body = t("inbox.maintenance.body", { count: n.data!.count });
+    body = t("inbox.maintenance.body", { count: d.count });
   } else if (isScheduler) {
     // "<Job label> finished/failed", with the raw result summary as the (technical) body.
-    const job = t(`scheduler.jobs.${n.data!.job_id}`, n.data!.job_id);
-    title = t(n.data!.status === "error" ? "inbox.jobDone.titleError" : "inbox.jobDone.titleOk", {
+    const d = n.data as { job_id: string; status?: string; summary?: string };
+    const job = t(`scheduler.jobs.${d.job_id}`, d.job_id);
+    title = t(d.status === "error" ? "inbox.jobDone.titleError" : "inbox.jobDone.titleOk", {
       job,
     });
-    body = n.data!.summary || n.body;
+    body = d.summary || n.body;
   }
   return (
     <div
@@ -320,7 +324,7 @@ function ClientActivityRow({
   onRemove,
 }: {
   n: ClientNotif;
-  t: (k: string, o?: any) => string;
+  t: TFunction;
   onFind: (meta: VideoHiddenMeta | VideoWatchedMeta) => void;
   onRevert: (meta: VideoHiddenMeta | VideoWatchedMeta) => void;
   onFocusChannel: (name: string) => void;