fix(ui): E31 S2 — the tab names the object, the channel header wraps, the player fits a short window

- The browser tab names the place and the object: PageHeading reports the
  view's own title (lib/documentTitle), so a channel, playlist or any Plex
  view reads "<object> · <module> · Siftlode" however it was opened; the
  Feed is named too; a season adds its show (tabTitle).
- The channel page's tab + action row wraps and lets the tag lane fold into
  "+N": at 1024 it ran past the edge and the page scrolled sideways.
- The YouTube player card's width is capped from the window height less the
  chrome measured under the video: at 133% zoom (685px) it cut its channel row.
- Plex certificate words (Not Rated, Approved, Passed) are named through the
  vocabulary on the info badge, show page, collection, filter and chips.
- DESIGN.md: the zoom target is 125% and 133%, not 200% (decision log).

frontend/src/components/PlexSidebar.tsx

@@ -2,6 +2,7 @@ import { useState } from "react";
 import { qk } from "../lib/queryKeys";
 import { usePlexCollectionTitles } from "../lib/usePlexCollectionTitles";
 import { useTranslation } from "react-i18next";
+import { vocab } from "../lib/vocab";
 import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { ListMusic, Plus, SlidersHorizontal } from "lucide-react";
@@ -247,7 +248,10 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
             body: (
               <Combobox
                 label={t("plex.filter.contentRating")}
-                options={facets.content_ratings}
+                options={facets.content_ratings.map((o) => ({
+                  ...o,
+                  label: vocab(t, "certificate", o.value),
+                }))}
                 selected={filters.contentRatings}
                 onChange={(next) => patch({ contentRatings: next })}
               />