refactor(rail): extract shared FilterRail shell + name the MediaSource seam (E4 S7)

The three filter rails — feed Sidebar, PlexSidebar, PlaylistsRail — each rendered
the same SidePanel chrome (title bar, customize `editing` state, edit-toggle,
reset-to-layout-default, edit hint) around a PanelGroups body, differing only in
identity + filter content. Extracted that shell into components/FilterRail.tsx:
each rail now supplies only what differs (title/icon, active-filter count +
Clear-all action, its `items`, and optional content above/below the groups —
`top` for the feed's scope toggle, `children` for its tag manager). FilterRail
owns the `editing` state, so all three drop their duplicated useState + SidePanel
wiring.

MediaSource facade (lib/mediaSource.ts): a thin `MediaSourceKind = "feed" | "plex"`
naming the two browsable, filterable sources (a PanelId minus "playlists", which
is a rail but not a source). Already unified in practice — one SortControl, the
source-agnostic ActiveFilter[] chips, PanelId layouts, now the shared shell — so
this is formalization, not new capability: a single named seam for a future third
source or a cross-source feature. The two media-source rails annotate their `kind`
with it; PlaylistsRail passes the plain PanelId. Per the recon, the playlist-dialog
merge was deliberately NOT forced (it proved forced + low value). UX-3 control-vocab
sweep deliberately deferred — the rails already use the canonical primitives.

Behaviour-preserving. Gate green (tsc, knip, lint 0-err, 247 vitest). Self-E2E
(localdev): all three rails render via FilterRail — feed (scope toggle + panel
groups + Customize→editing with reset + 56 reorder affordances, exits clean), Plex
(filter groups + customize), Playlists (sections + customize); 0 JS console errors.

frontend/src/components/PlexSidebar.tsx

@@ -5,11 +5,12 @@ import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Layers, ListMusic, Plus, SlidersHorizontal, X } from "lucide-react";
 import { api, plexFilterCount, type PlexFilters } from "../lib/api";
-import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
+import { type PanelLayout } from "../lib/panelLayout";
+import { type MediaSourceKind } from "../lib/mediaSource";
 import { useDebounced } from "../lib/useDebounced";
 import { useScrollFade } from "../lib/useScrollFade";
-import SidePanel from "./SidePanel";
-import PanelGroups, { type PanelItem } from "./PanelGroups";
+import FilterRail from "./FilterRail";
+import { type PanelItem } from "./PanelGroups";
 import SegmentedToggle from "./SegmentedToggle";
 import FilterChipGroup, { filterChipClass } from "./FilterChipGroup";
 import { usePlexClearFilters } from "./usePlexActiveFilters";
@@ -70,7 +71,9 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   const collFade = useScrollFade();
   const playlistsQ = useQuery({ queryKey: qk.plexPlaylists(), queryFn: () => api.plexPlaylists() });
   const [newPlaylist, setNewPlaylist] = useState("");
-  const [editing, setEditing] = useState(false);
+  // This rail's media source (the Plex library) — names the abstraction the feed and Plex rails
+  // share; drives the panel layout key. `FilterRail` owns the customize `editing` state now.
+  const source: MediaSourceKind = "plex";
   const qc = useQueryClient();
   async function createPlaylist() {
     const title = newPlaylist.trim();
@@ -402,16 +405,16 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   ];
 
   return (
-    <SidePanel
+    <FilterRail
+      kind={source}
       title={t("sidebar.filters")}
       icon={<SlidersHorizontal className="w-4 h-4" />}
       count={activeCount}
       collapsed={collapsed}
       onToggleCollapse={onToggleCollapse}
-      editing={editing}
-      onToggleEditing={() => setEditing((e) => !e)}
-      onReset={() => setLayout(defaultLayout("plex"))}
-      editHint={t("sidebar.editHint")}
+      layout={layout}
+      setLayout={setLayout}
+      items={items}
       actions={
         anyActive ? (
           <button
@@ -423,9 +426,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
           </button>
         ) : undefined
       }
-    >
-      <PanelGroups items={items} layout={layout} setLayout={setLayout} editing={editing} />
-    </SidePanel>
+    />
   );
 }