chore(downloads-ui): FE-C4 follow-up cleanup

- Delete dead top-level downloads.rename.* i18n block (en/hu/de); the
  metadata modal moved to downloads.edit.*. Keep downloads.actions.rename
  (still used by ProfileEditor).
- Rename misnamed state renameJob/setRenameJob -> metaJob/setMetaJob in
  DownloadCenter (it opens MetaEditModal, not a rename dialog).
- Extract the duplicated [downloads]/[download-index]/[download-usage]
  invalidation trio into lib/downloads.ts invalidateDownloads(qc);
  used by DownloadCenter and DownloadDialog.

Behavior-neutral. tsc green, knip shows no new unused export, JSON valid.

frontend/src/components/DownloadCenter.tsx

@@ -27,6 +27,7 @@ const ShareDialog = lazy(() => import("./ShareDialog"));
 import { useConfirm } from "./ConfirmProvider";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { api, type DownloadJob, type Me } from "../lib/api";
+import { invalidateDownloads } from "../lib/downloads";
 import { notify } from "../lib/notifications";
 import { formatBytes, formatDuration, formatEta, formatSpeed } from "../lib/format";
 
@@ -546,7 +547,7 @@ export default function DownloadCenter({ me }: { me: Me }) {
   const [addUrl, setAddUrl] = useState("");
   const [addSource, setAddSource] = useState<string | null>(null);
   const [manage, setManage] = useState(false);
-  const [renameJob, setRenameJob] = useState<DownloadJob | null>(null);
+  const [metaJob, setMetaJob] = useState<DownloadJob | null>(null);
   const [shareJob, setShareJob] = useState<DownloadJob | null>(null);
   const [editJob, setEditJob] = useState<DownloadJob | null>(null);
 
@@ -556,11 +557,7 @@ export default function DownloadCenter({ me }: { me: Me }) {
   const queue = jobs.filter((j) => ["queued", "running", "paused", "error"].includes(j.status));
   const library = jobs.filter((j) => j.status === "done");
 
-  const invalidate = () => {
-    qc.invalidateQueries({ queryKey: ["downloads"] });
-    qc.invalidateQueries({ queryKey: ["download-index"] });
-    qc.invalidateQueries({ queryKey: ["download-usage"] });
-  };
+  const invalidate = () => invalidateDownloads(qc);
   const act = useMutation({
     mutationFn: ({ id, op }: { id: number; op: "pause" | "resume" | "cancel" | "delete" }) =>
       op === "pause" ? api.pauseDownload(id)
@@ -674,7 +671,7 @@ export default function DownloadCenter({ me }: { me: Me }) {
                     <Scissors className="w-4 h-4" />
                   </IconBtn>
                 )}
-                <IconBtn onClick={() => setRenameJob(job)} title={t("downloads.actions.editDetails")}>
+                <IconBtn onClick={() => setMetaJob(job)} title={t("downloads.actions.editDetails")}>
                   <Pencil className="w-4 h-4" />
                 </IconBtn>
                 <IconBtn onClick={() => setShareJob(job)} title={t("downloads.actions.share")}>
@@ -738,7 +735,7 @@ export default function DownloadCenter({ me }: { me: Me }) {
           />
         )}
         {manage && <ProfileEditor onClose={() => setManage(false)} />}
-        {renameJob && <MetaEditModal job={renameJob} onClose={() => setRenameJob(null)} />}
+        {metaJob && <MetaEditModal job={metaJob} onClose={() => setMetaJob(null)} />}
         {shareJob && <ShareDialog job={shareJob} onClose={() => setShareJob(null)} />}
         {editJob && <VideoEditor job={editJob} onClose={() => setEditJob(null)} />}
       </Suspense>

frontend/src/components/DownloadDialog.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Modal from "./Modal";
 import { api } from "../lib/api";
+import { invalidateDownloads } from "../lib/downloads";
 
 // Lazy: the full profile editor only opens from the dialog's "manage presets" affordance.
 const ProfileEditor = lazy(() => import("./ProfileEditor"));
@@ -43,9 +44,7 @@ export default function DownloadDialog({
         profile_id: chosen,
         display_name: name.trim() || undefined,
       });
-      qc.invalidateQueries({ queryKey: ["downloads"] });
-      qc.invalidateQueries({ queryKey: ["download-index"] });
-      qc.invalidateQueries({ queryKey: ["download-usage"] });
+      invalidateDownloads(qc);
       notify({
         level: "success",
         message: t("downloads.dialog.added"),