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/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",