feat(plex): port the filter bar to the YT-feed pattern

Bring the Plex module's filter surface in line with the feed's final shape:

- Flatten the rail: PlexSidebar passes `collapsible={false}` to FilterRail so
  the widgets become plain always-open label + control rows (no collapse
  chevrons; reorder/hide in edit mode stay). Drop the rail's own "Clear all" —
  the chip row above the grid already owns it.
- Move Sort + the Discover/Browse toggle into the applied-filter chip row,
  pinned right directly above the grid (a `sortView` fragment, like the feed's),
  and give ActiveFilterChips `flex-1` so it gets the shared responsive overflow
  (wide: fit + "+N" pill; narrow: single "N filters" count pill) — the chips can
  no longer crowd the controls off-screen.

ItemCount leads its own toolbar row, matching the feed. No behaviour change to
Discover, which keeps its standalone view toggle.

frontend/src/components/PlexSidebar.tsx

@@ -13,7 +13,6 @@ import { type PanelItem } from "./PanelGroups";
 import SegmentedToggle from "./SegmentedToggle";
 import FilterChipGroup from "./FilterChipGroup";
 import Combobox, { type ComboboxOption } from "./Combobox";
-import { usePlexClearFilters } from "./usePlexActiveFilters";
 import { usePlex, usePlexActions } from "./PlexProvider";
 
 // The Plex module's left filter column — now the shared floating SidePanel with its filter groups
@@ -96,10 +95,8 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   // Scope and sort moved to the header / above-the-grid toolbar (S3); the panel badge now counts only
   // the filters that still live in this panel (expanded filters + watch-state).
   const activeCount = fCount + (show !== "all" ? 1 : 0);
-  const anyActive = activeCount > 0;
 
   const patch = (p: Partial<PlexFilters>) => setFilters({ ...filters, ...p });
-  const clearAll = usePlexClearFilters();
 
   const durBucketKey = DURATION_BUCKETS.find(
     (b) => (filters.durationMin ?? null) === b.min && (filters.durationMax ?? null) === b.max,
@@ -361,17 +358,9 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
       layout={layout}
       setLayout={setLayout}
       items={items}
-      actions={
-        anyActive ? (
-          <button
-            onClick={clearAll}
-            className="inline-flex items-center gap-1 text-xs text-muted hover:text-accent transition px-1 whitespace-nowrap"
-          >
-            <X className="w-3.5 h-3.5" />
-            {t("plex.filter.clearAll")}
-          </button>
-        ) : undefined
-      }
+      // The Plex widgets are simple label + control rows now, not collapsible islands (matches the
+      // feed rail). The rail's own "Clear all" is dropped too — the chip row above the grid owns it.
+      collapsible={false}
     />
   );
 }