Fix Library Scan tab race condition mixing results across kinds

Switching tabs quickly (e.g. Movies -> Music) had no guard against an
in-flight response from the previous tab resolving late and overwriting
the new tab's results — showing movie/TV rips under the Music tab. Now
tracks a request sequence number and ignores stale responses, and clears
results immediately on tab switch so nothing renders under the wrong label.

frontend/src/pages/LibraryScan.jsx

@@ -1,4 +1,4 @@
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import api from "../lib/api";
 import { toast } from "sonner";
 import { RefreshCcw, Download, Check, Sparkles, CheckSquare, Square } from "lucide-react";
@@ -16,21 +16,25 @@ export default function LibraryScan() {
   const [loading, setLoading] = useState(false);
   const [importing, setImporting] = useState(false);
   const [enrich, setEnrich] = useState(null);
+  const requestSeq = useRef(0);
 
   const load = async (k = kind) => {
+    const seq = ++requestSeq.current;
     setLoading(true);
     setSelected(new Set());
     try {
       const { data } = await api.get(`/scan/${k}`);
+      if (seq !== requestSeq.current) return; // a newer tab switch/rescan superseded this response
       setResults(data);
     } catch (err) {
+      if (seq !== requestSeq.current) return;
       toast.error(err.response?.data?.detail || "Scan failed");
     } finally {
-      setLoading(false);
+      if (seq === requestSeq.current) setLoading(false);
     }
   };
 
-  useEffect(() => { load(kind); }, [kind]);
+  useEffect(() => { setResults([]); load(kind); }, [kind]);
 
   useEffect(() => {
     if (!enrich?.running) return;