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/PlayerModal.tsx

@@ -30,6 +30,7 @@ import AddToPlaylist from "./AddToPlaylist";
 import { modalCount } from "./Modal";
 import DownloadButton from "./DownloadButton";
 import { api, type Video, type VideoStatus } from "../lib/api";
+import type { YTNamespace, YTPlayer, YTPlayerEvent } from "../lib/youtube";
 import {
   channelYouTubeUrl,
   formatDate,
@@ -82,16 +83,17 @@ function canScrollBy(el: HTMLElement | null, dir: -1 | 1): boolean {
 }
 
 // --- IFrame Player API loader (singleton) ---
-let apiPromise: Promise<any> | null = null;
-function loadYouTubeApi(): Promise<any> {
+let apiPromise: Promise<YTNamespace> | null = null;
+function loadYouTubeApi(): Promise<YTNamespace> {
   if (apiPromise) return apiPromise;
-  apiPromise = new Promise((resolve) => {
-    const w = window as any;
+  apiPromise = new Promise<YTNamespace>((resolve) => {
+    const w = window;
     if (w.YT && w.YT.Player) return resolve(w.YT);
     const prev = w.onYouTubeIframeAPIReady;
     w.onYouTubeIframeAPIReady = () => {
       if (typeof prev === "function") prev();
-      resolve(w.YT);
+      // The API-ready callback only fires once the script has installed window.YT.
+      resolve(w.YT!);
     };
     const tag = document.createElement("script");
     tag.src = "https://www.youtube.com/iframe_api";
@@ -158,7 +160,7 @@ export default function PlayerModal({
   const resumeAt =
     startAt != null && active.id === video.id ? startAt : active.position_seconds || 0;
   const mountRef = useRef<HTMLDivElement | null>(null);
-  const playerRef = useRef<any>(null);
+  const playerRef = useRef<YTPlayer | null>(null);
   const autoMarkedRef = useRef(false);
   // Bound consecutive unplayable items so an auto-advancing queue (esp. loop=all) can't spin over a
   // run of dead videos — after the cap we stop on the error overlay instead of skipping forever.
@@ -706,7 +708,7 @@ export default function PlayerModal({
             focusModal();
             applyVolume(volumeRef.current, false);
           },
-          onStateChange: (e: any) => {
+          onStateChange: (e: YTPlayerEvent) => {
             // 1 === playing → sync the navigated-to video's title/author for display.
             if (e?.data === 1) {
               errorStreakRef.current = 0; // a successful play breaks any unplayable run
@@ -725,7 +727,7 @@ export default function PlayerModal({
           },
           // The embed couldn't play this video (embedding disabled, removed, private…).
           // Surface our own message + an "Open on YouTube" escape hatch.
-          onError: (e: any) => {
+          onError: (e: YTPlayerEvent) => {
             setPlayerError(typeof e?.data === "number" ? e.data : -1);
             // The IFrame API fires onError (not 'ended') for an unplayable item, so advanceOnEnd()
             // never runs and an auto-advancing queue would freeze here. Skip forward instead — but