feat(header): S7 — container-query fix for the narrow Plex header

The floating header's search box collapsed to ~0 width when the filter sidebar
was open on a narrow window (~768px): the content column shrinks with the
SIDEBAR, not the viewport, so Tailwind viewport breakpoints can't see it, and
SearchBar's `flex-1 min-w-0` shrank rather than wrapped.

Fix with Tailwind v4 container queries (no plugin):
- App.tsx: mark the content column `@container/appmain` (container-type:
  inline-size) — the width that actually shrinks when the sidebar opens.
- SearchBar: once the column has room (≥420px) enforce a `min-w-[15rem]` floor
  so the box WRAPS to its own line via the header's flex-wrap instead of
  collapsing the input; on a narrow column (≤640px) it takes a full line of its
  own (basis-full) and sorts last, so the small pills (module name + Plex scope
  toggle / feed sync chip) share the line above it — a tidy two-row header.

Verified live at 375 / 768 / 1024 / 1280 on Plex (search usable, scope toggle
reachable at every width) and on Feed (shared SearchBar, no regression). Desktop
stays a single row. tsc / knip / lint clean.

frontend/src/App.tsx

@@ -351,7 +351,10 @@ export default function App() {
           />
         </Suspense>
       )}
-      <div className="relative flex-1 min-w-0 flex flex-col">
+      {/* `@container/appmain`: the floating header + its search box adapt to THIS column's width
+          (which shrinks when the filter sidebar is open), not the viewport — a viewport breakpoint
+          can't see the sidebar-driven narrowing (E5 S7). */}
+      <div className="@container/appmain relative flex-1 min-w-0 flex flex-col">
         {channelView ? (
           <PageShell scrollKey={`channel:${channelView.id}`}>
             <Suspense fallback={pageFallback}>