feat(downloads): S1 UI quick wins — queue default, dates, auto-link, thumb fallback

- Always land on the Queue tab (drop the persisted last-tab) so new work/failures are the default view
- Show the download date (relative) on Library rows
- Share dialog opens with a default link already generated when none exists; all link controls stay available
- Thumbnail falls back to the self-hosted poster when a remote (i.ytimg/fbcdn) thumbnail 404s
- Add a clipboard-paste affordance to the add-URL box (explicit button + best-effort on focus)

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

@@ -20,6 +20,7 @@
     "subtitle": "Download videos to the server, then save them to your device.",
     "addUrl": "Paste a YouTube link or video id…",
     "add": "Add",
+    "paste": "Paste link from clipboard",
     "manage": "Manage formats"
   },
   "tabs": {

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

@@ -20,6 +20,7 @@
     "subtitle": "Töltsd le a videókat a szerverre, majd mentsd a saját gépedre.",
     "addUrl": "Illessz be egy YouTube linket vagy videó azonosítót…",
     "add": "Hozzáad",
+    "paste": "Link beillesztése a vágólapról",
     "manage": "Formátumok kezelése"
   },
   "tabs": {