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/PlexPlayer.tsx

@@ -34,7 +34,7 @@ import {
   VolumeX,
   X,
 } from "lucide-react";
-import { api, type PlexMarker } from "../lib/api";
+import { api, HttpError, type PlexMarker } from "../lib/api";
 import { formatDuration } from "../lib/format";
 import {
   exitFullscreen,
@@ -314,14 +314,15 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
         // selected client-side after the manifest loads.
         const m = multiAudioRef.current;
         sess = await api.plexSession(id, startAt, m ? null : audioRef.current, aoffRef.current, m);
-      } catch (e: any) {
+      } catch (e) {
         // Surface WHY playback can't start instead of an eternal spinner. 404 = the physical file
         // isn't reachable (missing media bind-mount or a wrong plex_path_map); 501 = the codec
         // needs full transcoding (a later phase).
+        const status = e instanceof HttpError ? e.status : undefined;
         setLoadError(
-          e?.status === 501
+          status === 501
             ? tRef.current("plex.player.errTranscode")
-            : e?.status === 404
+            : status === 404
               ? tRef.current("plex.player.errNotFound")
               : tRef.current("plex.player.errGeneric"),
         );