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/Header.tsx

@@ -94,7 +94,7 @@ export default function Header({
     // Absolutely positioned within the (relative) content column, so its left edge tracks the
     // actual left zone: it sits just inside the content, reclaiming the space when the nav rail is
     // slim and/or no filter sidebar is present — no hard-coded offset.
-    <div className="pointer-events-none absolute top-3 left-3 right-[10%] z-chrome flex items-center gap-3">
+    <div className="pointer-events-none absolute top-3 left-3 right-[10%] z-chrome flex flex-wrap items-center gap-x-3 gap-y-2">
       <ModuleName
         label={t(moduleLabelKey[page])}
         labels={moduleLabels}