feat(downloads): let admins set a quota for any user, not only downloaders

The admin System tab only listed users with an existing footprint, so a quota could be set only
after someone downloaded something. Add a 'Set a user's quota…' picker (over /api/admin/users,
excl. demo) beside the Per-user footprint heading that opens the quota editor for ANY user — the
quota GET/PUT endpoints already resolve defaults + create a row on demand, so no backend change.
Also show an empty-state line when no one has a footprint yet. i18n en/hu/de.

frontend/src/components/DownloadCenter.tsx

@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -275,6 +275,56 @@ function QuotaModal({ userId, email, onClose }: { userId: number; email: string;
   );
 }
 
+// Pick ANY user to set a download quota for — even one who hasn't downloaded anything yet (the
+// footprint list below only shows users who already have files).
+function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }) => void }) {
+  const { t } = useTranslation();
+  const [q, setQ] = useState("");
+  const [open, setOpen] = useState(false);
+  const users = useQuery({ queryKey: ["admin-users"], queryFn: api.adminUsers });
+  const list = (users.data ?? []).filter((u) => !u.is_demo);
+  const filtered = useMemo(() => {
+    const s = q.trim().toLowerCase();
+    const base = s
+      ? list.filter((u) => u.email.toLowerCase().includes(s) || (u.display_name ?? "").toLowerCase().includes(s))
+      : list;
+    return base.slice(0, 8);
+  }, [q, list]);
+  return (
+    <div className="relative w-full sm:w-72">
+      <input
+        value={q}
+        onChange={(e) => {
+          setQ(e.target.value);
+          setOpen(true);
+        }}
+        onFocus={() => setOpen(true)}
+        onBlur={() => setTimeout(() => setOpen(false), 150)}
+        placeholder={t("downloads.admin.quotaPickPlaceholder")}
+        className={inputCls}
+      />
+      {open && filtered.length > 0 && (
+        <div className="absolute z-10 mt-1 w-full rounded-lg border border-border bg-card shadow-xl overflow-hidden">
+          {filtered.map((u) => (
+            <button
+              key={u.id}
+              onClick={() => {
+                onPick({ id: u.id, email: u.email });
+                setQ("");
+                setOpen(false);
+              }}
+              className="w-full text-left px-3 py-2 text-sm hover:bg-surface transition flex flex-col"
+            >
+              <span className="truncate">{u.display_name || u.email}</span>
+              {u.display_name && <span className="text-xs text-muted truncate">{u.email}</span>}
+            </button>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}
+
 function AdminSystem() {
   const { t } = useTranslation();
   const storage = useLiveQuery(["admin-storage"], api.adminDownloadStorage, { intervalMs: 5000 });
@@ -298,9 +348,13 @@ function AdminSystem() {
         </div>
       )}
 
-      {s && s.per_user.length > 0 && (
+      {s && (
         <div className="mb-6">
-          <div className="text-sm font-medium mb-2">{t("downloads.admin.perUser")}</div>
+          <div className="flex items-center justify-between gap-3 mb-2 flex-wrap">
+            <div className="text-sm font-medium">{t("downloads.admin.perUser")}</div>
+            {/* Set a quota for any user, regardless of whether they've downloaded anything. */}
+            <QuotaUserPicker onPick={setQuotaFor} />
+          </div>
           <div className="space-y-1">
             {s.per_user.map((u) => (
               <div key={u.user_id} className="flex items-center gap-2 text-sm px-3 py-1.5 rounded-lg bg-card/40">
@@ -315,6 +369,9 @@ function AdminSystem() {
                 </button>
               </div>
             ))}
+            {s.per_user.length === 0 && (
+              <div className="text-xs text-muted px-3 py-2">{t("downloads.admin.noFootprint")}</div>
+            )}
           </div>
         </div>
       )}

frontend/src/i18n/locales/de/downloads.json

@@ -157,7 +157,9 @@
     "reset": "Auf Standard zurücksetzen",
     "save": "Speichern",
     "custom": "individuell",
-    "default": "Standard"
+    "default": "Standard",
+    "quotaPickPlaceholder": "Kontingent für einen Nutzer festlegen…",
+    "noFootprint": "Noch keine Downloads."
   },
   "cols": {
     "title": "Titel",

frontend/src/i18n/locales/en/downloads.json

@@ -157,7 +157,9 @@
     "reset": "Reset to default",
     "save": "Save",
     "custom": "custom",
-    "default": "default"
+    "default": "default",
+    "quotaPickPlaceholder": "Set a user's quota…",
+    "noFootprint": "No downloads yet."
   },
   "cols": {
     "title": "Title",

frontend/src/i18n/locales/hu/downloads.json

@@ -157,7 +157,9 @@
     "reset": "Visszaállítás alapértékre",
     "save": "Mentés",
     "custom": "egyedi",
-    "default": "alapértelmezett"
+    "default": "alapértelmezett",
+    "quotaPickPlaceholder": "Kvóta beállítása egy felhasználónak…",
+    "noFootprint": "Még nincs letöltés."
   },
   "cols": {
     "title": "Cím",