fix(plex): address S3 review nits — tests + narrow-header wrap

- Add PlexScopeToggle.dom.test.tsx (renders the three scopes, current one
  pressed, click writes scope back to the provider) and
  usePlexActiveFilters.dom.test.tsx (emits filter + watch-state chips but never
  a sort chip, even with a non-default sort active) — the S3 relocation was
  untested.
- Narrow-width header: make the header row flex-wrap so the scope toggle drops
  to a second line instead of crowding the flex-1 search box to zero width, and
  let the field-scope pill shrink/truncate. Desktop stays a single line
  (verified: 1280px header unchanged, search 237px, no wrap).

The removed one-click sort-reset chip is intended (sort is an always-visible
control now, like the Feed) — no change. FE suite 174 green; typecheck + eslint
clean.

frontend/src/components/PlexSearchBox.tsx

@@ -117,10 +117,10 @@ export default function PlexSearchBox() {
       leading={
         <ActionMenu
           ariaLabel={t("plex.searchField.aria")}
-          className="shrink-0 inline-flex items-center gap-1 rounded-lg px-2 h-7 text-xs font-medium text-muted transition hover:text-fg hover:bg-border/50"
+          className="min-w-0 shrink inline-flex items-center gap-1 rounded-lg px-2 h-7 text-xs font-medium text-muted transition hover:text-fg hover:bg-border/50"
           trigger={
             <>
-              <span className="max-w-28 truncate">{pillLabel}</span>
+              <span className="min-w-0 max-w-28 truncate">{pillLabel}</span>
               <ChevronDown className="w-3 h-3 shrink-0" aria-hidden="true" />
             </>
           }