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/components/Sidebar.tsx

@@ -8,6 +8,7 @@ import {
   EyeOff,
   GripVertical,
   Pencil,
+  RefreshCw,
   RotateCcw,
   X,
 } from "lucide-react";
@@ -48,6 +49,9 @@ const SORT_IDS = [
 
 const SHOW_IDS = ["unwatched", "in_progress", "all", "watched", "saved", "hidden"];
 
+// Fresh shuffle token for the "surprise me" sort.
+const rollSeed = () => Math.floor(Math.random() * 1_000_000_000);
+
 const DATE_PRESETS: { days: number; key: string }[] = [
   { days: 1, key: "24h" },
   { days: 7, key: "1week" },
@@ -215,17 +219,38 @@ export default function Sidebar({
         );
       case "sort":
         return (
-          <select
-            value={filters.sort}
-            onChange={(e) => setFilters({ ...filters, sort: e.target.value })}
-            className="w-full bg-card border border-border rounded-lg px-2 py-1.5 text-sm outline-none focus:border-accent"
-          >
-            {SORT_IDS.map((id) => (
-              <option key={id} value={id}>
-                {t("sidebar.sort." + id)}
-              </option>
-            ))}
-          </select>
+          <div className="flex items-center gap-1.5">
+            <select
+              value={filters.sort}
+              onChange={(e) => {
+                const sort = e.target.value;
+                // Seed shuffle on selection so it lands on a fresh order, not the
+                // deterministic seed-0 one every time.
+                setFilters({
+                  ...filters,
+                  sort,
+                  seed: sort === "shuffle" ? rollSeed() : undefined,
+                });
+              }}
+              className="w-full bg-card border border-border rounded-lg px-2 py-1.5 text-sm outline-none focus:border-accent"
+            >
+              {SORT_IDS.map((id) => (
+                <option key={id} value={id}>
+                  {t("sidebar.sort." + id)}
+                </option>
+              ))}
+            </select>
+            {filters.sort === "shuffle" && (
+              <button
+                onClick={() => setFilters({ ...filters, seed: rollSeed() })}
+                title={t("sidebar.reshuffle")}
+                aria-label={t("sidebar.reshuffle")}
+                className="shrink-0 p-1.5 rounded-lg border border-border bg-card text-fg shadow-sm hover:border-accent hover:text-accent active:translate-y-px transition"
+              >
+                <RefreshCw className="w-4 h-4" />
+              </button>
+            )}
+          </div>
         );
       case "date":
         return (