feat(feed): shared-library scope toggle in the header

Add a "Mine / Library" segmented toggle (shown on the feed page) that switches
FeedFilters.scope between the user's own subscriptions and the whole shared
catalog. A read-scope-less user (signed in but no YouTube grant) can now browse
and manage the shared library on their own account — the empty "my feed" state
offers a "browse the shared library" shortcut alongside the connect-YouTube CTA.
scope is preserved across "Clear all" (it's a mode, not a filter) and kept out
of the URL state. Trilingual strings (HU/EN/DE) for the toggle and the CTA.

frontend/src/components/Feed.tsx

@@ -148,7 +148,9 @@ export default function Feed({
   if (query.isLoading) return <div className="p-8 text-muted">{t("feed.loading")}</div>;
   if (query.isError) return <div className="p-8 text-muted">{t("feed.loadError")}</div>;
   if (items.length === 0) {
-    if (!canRead)
+    // No YouTube connection and looking at their own (empty) subscriptions: offer both
+    // connecting YouTube and just browsing the shared library (no read scope needed).
+    if (!canRead && filters.scope === "my")
       return (
         <div className="p-8 grid place-items-center text-center">
           <div className="max-w-sm">
@@ -160,6 +162,12 @@ export default function Feed({
             >
               {t("feed.setUp")}
             </button>
+            <button
+              onClick={() => setFilters({ ...filters, scope: "all" })}
+              className="block mx-auto mt-3 text-sm text-muted hover:text-accent transition"
+            >
+              {t("feed.browseShared")}
+            </button>
           </div>
         </div>
       );

frontend/src/components/Header.tsx

@@ -1,6 +1,6 @@
 import { useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
-import { BarChart3, Home, Info, LogOut, Search, Settings, Shield, Tv } from "lucide-react";
+import { BarChart3, Home, Info, Library, LogOut, Search, Settings, Shield, Tv, User } from "lucide-react";
 import type { FeedFilters, Me } from "../lib/api";
 import type { Page } from "../lib/urlState";
 import { type LangCode } from "../i18n";
@@ -49,6 +49,35 @@ export default function Header({
 
       <SyncStatus isAdmin={me.role === "admin"} onGoToFullHistory={onGoToFullHistory} />
 
+      {page === "feed" && (
+        <div
+          className="flex items-center rounded-full border border-border bg-card p-0.5 text-xs shrink-0"
+          role="group"
+          aria-label={t("header.scope.label")}
+        >
+          {(["my", "all"] as const).map((s) => (
+            <button
+              key={s}
+              onClick={() => setFilters({ ...filters, scope: s })}
+              title={t("header.scope." + s + "Tip")}
+              aria-pressed={filters.scope === s}
+              className={`inline-flex items-center gap-1 px-2.5 py-1 rounded-full transition ${
+                filters.scope === s
+                  ? "bg-accent text-accent-fg"
+                  : "text-muted hover:text-fg"
+              }`}
+            >
+              {s === "my" ? (
+                <User className="w-3.5 h-3.5" />
+              ) : (
+                <Library className="w-3.5 h-3.5" />
+              )}
+              <span className="hidden sm:inline">{t("header.scope." + s)}</span>
+            </button>
+          ))}
+        </div>
+      )}
+
       {page === "feed" ? (
         <div className="flex-1 max-w-xl mx-auto relative">
           <Search className="w-4 h-4 absolute left-3 top-1/2 -translate-y-1/2 text-muted" />

frontend/src/components/Sidebar.tsx

@@ -60,8 +60,9 @@ const DATE_PRESETS: { days: number; key: string }[] = [
   { days: 365, key: "1year" },
 ];
 
-// Filter values owned by the sidebar (everything except the header search `q`).
-const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q"> = {
+// Filter values owned by the sidebar (everything except the header search `q` and the
+// `scope` mode, both preserved across a "Clear all").
+const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q" | "scope"> = {
   tags: [],
   tagMode: "or",
   sort: "newest",
@@ -170,7 +171,7 @@ export default function Sidebar({
 
   function clearAll() {
     setCustomDates(false);
-    setFilters({ ...DEFAULT_SIDEBAR_FILTERS, q: filters.q });
+    setFilters({ ...DEFAULT_SIDEBAR_FILTERS, q: filters.q, scope: filters.scope });
   }
 
   const available: Record<WidgetId, boolean> = {