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/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" />