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/lib/youtube.ts

@@ -0,0 +1,59 @@
+// Minimal typings for the YouTube IFrame Player API surface this app actually uses. The API is
+// loaded at runtime from youtube.com/iframe_api and ships no official npm types; rather than pull in
+// @types/youtube we declare just the methods/events PlayerModal calls, so tsc flags any new call
+// that isn't covered here. See [[reference-yt-iframe-api-remote-control-map]] for what's driveable.
+
+export interface YTVideoData {
+  title: string;
+  author: string;
+  video_id?: string;
+}
+
+export interface YTPlayer {
+  getPlayerState(): number;
+  getCurrentTime(): number;
+  getDuration(): number;
+  getVideoData(): YTVideoData;
+  getIframe(): HTMLIFrameElement;
+  getVolume(): number;
+  isMuted(): boolean;
+  setVolume(volume: number): void;
+  mute(): void;
+  unMute(): void;
+  playVideo(): void;
+  pauseVideo(): void;
+  loadVideoById(opts: { videoId: string; startSeconds?: number }): void;
+  seekTo(seconds: number, allowSeekAhead: boolean): void;
+  destroy(): void;
+}
+
+// The IFrame API fires events carrying a numeric `data` — a player-state code (onStateChange) or an
+// error code (onError).
+export interface YTPlayerEvent {
+  data: number;
+  target: YTPlayer;
+}
+
+export interface YTPlayerOptions {
+  width?: string | number;
+  height?: string | number;
+  videoId?: string;
+  playerVars?: Record<string, string | number | undefined>;
+  events?: {
+    onReady?: (e: YTPlayerEvent) => void;
+    onStateChange?: (e: YTPlayerEvent) => void;
+    onError?: (e: YTPlayerEvent) => void;
+  };
+}
+
+export interface YTNamespace {
+  Player: new (el: HTMLElement, opts: YTPlayerOptions) => YTPlayer;
+}
+
+// The globals the IFrame API script installs on `window` once loaded.
+declare global {
+  interface Window {
+    YT?: YTNamespace;
+    onYouTubeIframeAPIReady?: () => void;
+  }
+}