feat(feed): reshuffle button for the "surprise me" sort

The backend shuffle sort already accepts a seed param. Add a circular
reshuffle button next to the sort control (shown only when shuffle is
active) that re-rolls the seed and re-queries the feed; selecting shuffle
also seeds a fresh order instead of the deterministic seed-0 one. The seed
lives in FeedFilters but is intentionally kept out of the URL state.

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

@@ -23,6 +23,7 @@
   "from": "Von",
   "to": "Bis",
   "clearDates": "Daten löschen",
+  "reshuffle": "Neu mischen",
   "widget": {
     "show": "Anzeigen",
     "sort": "Sortierung",