feat(feed): move the Mine/Library scope toggle beside the header search box

The scope toggle used to sit at the top of the filter rail, which is buried when
the rail is collapsed/unpinned. Relocate it to the header, just right of the feed
search box — mirroring Plex's scope toggle placement — so the primary 'what am I
browsing' control is always visible.

New FeedScopeToggle adapter (SegmentedToggle over filters.scope, same as the Plex
one over its scope); rendered in Header on the feed page between the search box and
the sync chip. Removed the toggle from the Sidebar rail's now-unused `top` slot and
dropped the dead `top` prop from FilterRail. Same filters.scope field, so behaviour
is unchanged.

Verified live: the toggle sits beside the feed search and switches scope; the rail
no longer carries it.

frontend/src/components/Sidebar.tsx

@@ -2,7 +2,7 @@ import { type ReactNode, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
-import { Library, Pencil, SlidersHorizontal, User } from "lucide-react";
+import { Pencil, SlidersHorizontal } from "lucide-react";
 import { api, type FeedFilters, type Tag } from "../lib/api";
 import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
 import { useDebounced } from "../lib/useDebounced";
@@ -283,30 +283,6 @@ export default function Sidebar({
       setLayout={setLayout}
       items={items}
       actions={actions}
-      top={
-        /* Scope: your own subscriptions (Mine) vs the shared Library. */
-        <SegmentedToggle
-          ariaLabel={t("header.scope.label")}
-          fullWidth
-          size="md"
-          options={[
-            {
-              value: "my",
-              label: t("header.scope.my"),
-              icon: <User className="w-3.5 h-3.5" />,
-              title: t("header.scope.myTip"),
-            },
-            {
-              value: "all",
-              label: t("header.scope.all"),
-              icon: <Library className="w-3.5 h-3.5" />,
-              title: t("header.scope.allTip"),
-            },
-          ]}
-          value={filters.scope}
-          onChange={(scope) => setFilters({ ...filters, scope })}
-        />
-      }
     >
       {tagManagerOpen && (
         <TagManager onClose={() => setTagManagerOpen(false)} onFocusChannel={onFocusChannel} />