feat(downloads): local-upload UI — drag-drop dialog + progress (E7 S5c)

UploadDialog (drag-drop + native picker + optional display name) opened
from a new Upload button in the queue add-row. Uploads stream through a
new XHR helper (uploadReq in api/core) so the dialog shows real 0-100%
progress with a cancel — fetch/req can't report upload progress. The
upload lands as an already-done job; the S5b file-kind row gates it to
download+share. i18n HU/EN; upload cap/empty errors localize via the
shared structured-detail path.

frontend/src/components/DownloadCenter.tsx

@@ -17,6 +17,7 @@ import {
   Share2,
   Pencil,
   Trash2,
+  UploadCloud,
 } from "lucide-react";
 import Tabs from "./Tabs";
 import { PageToolbar } from "./PageShell";
@@ -24,6 +25,7 @@ import { LoadingState, StateMessage } from "./QueryState";
 // Lazy: these dialogs/editors open on demand, so they stay out of the Downloads page chunk until
 // used (the video editor in particular is heavy — filmstrip + scrubber).
 const DownloadDialog = lazy(() => import("./DownloadDialog"));
+const UploadDialog = lazy(() => import("./UploadDialog"));
 const ProfileEditor = lazy(() => import("./ProfileEditor"));
 const VideoEditor = lazy(() => import("./VideoEditor"));
 const ShareDialog = lazy(() => import("./ShareDialog"));
@@ -49,6 +51,7 @@ export default function DownloadCenter() {
   const [tab, setTab] = useState("queue");
   const [addUrl, setAddUrl] = useState("");
   const [addSource, setAddSource] = useState<string | null>(null);
+  const [upload, setUpload] = useState(false);
   const [manage, setManage] = useState(false);
   const [metaJob, setMetaJob] = useState<DownloadJob | null>(null);
   const [shareJob, setShareJob] = useState<DownloadJob | null>(null);
@@ -207,6 +210,16 @@ export default function DownloadCenter() {
               >
                 {t("downloads.page.add")}
               </Button>
+              {/* Local upload is a peer to "add a URL": a general file → the same library (E7 S5c). */}
+              <Button
+                variant="outline"
+                size="sm"
+                icon={<UploadCloud className="w-4 h-4" />}
+                onClick={() => setUpload(true)}
+                className="shrink-0"
+              >
+                {t("downloads.page.upload")}
+              </Button>
             </div>
             <div className="space-y-2">
               {queue.map((job) => (
@@ -406,6 +419,7 @@ export default function DownloadCenter() {
               }}
             />
           )}
+          {upload && <UploadDialog onClose={() => setUpload(false)} />}
           {manage && <ProfileEditor onClose={() => setManage(false)} />}
           {metaJob && <MetaEditModal job={metaJob} onClose={() => setMetaJob(null)} />}
           {shareJob && <ShareDialog job={shareJob} onClose={() => setShareJob(null)} />}