fix(web): reuse speculative pre-decode on Şase Çöz instead of decoding twice

The 17-char speculative pre-decode fired a POST /vehicles/decode and discarded
the result, then the button click fired a second identical decode — two
requests per VIN. Keep the pre-decode promise and have runDecode reuse it on the
first attempt (awaiting it if still in flight), so the click navigates off the
already-running decode. Retries and the sub-350ms "clicked before warm" case
fall back to a fresh decode; a failed warm retries once.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/routes/dashboard/search.tsx

@@ -61,7 +61,9 @@ function SearchPage() {
   const candidatesShownAtRef = useRef<number | null>(null);
   const lastAttemptedVinRef = useRef<string | null>(null);
   const attemptCountRef = useRef<number>(0);
-  const preDecodedVinRef = useRef<string | null>(null);
+  // Holds the in-flight/finished speculative decode so the button can reuse it
+  // instead of firing a second identical request.
+  const preDecodeRef = useRef<{ vin: string; promise: Promise<any> } | null>(null);
   const [vin, setVin] = useState("");
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -149,33 +151,23 @@ function SearchPage() {
   }, [vin]);
 
   // ─── Speculative pre-decode (the moment the VIN is valid) ──────────────────
-  // Warm the full decode in the background before the user clicks "Şase Çöz", so
-  // the button-click decode hits the backend lock / 24h positive cache and returns
-  // near-instantly — the dealer at the counter loses no time. Fire-and-forget: no
-  // state, no analytics, no navigation (the button path owns all of that). Deduped
-  // per VIN, debounced + aborted so typing/paste-then-edit doesn't spam decodes.
+  // Decode the VIN in the background before the user clicks "Şase Çöz" and KEEP
+  // the result. The button click reuses this promise (see runDecode) rather than
+  // firing a second identical decode — one request, not two. Deduped per VIN and
+  // debounced so typing/paste-then-edit doesn't spam decodes.
   useEffect(() => {
     const cleanVin = vin.toUpperCase().trim();
-    if (!isValidVin(cleanVin) || preDecodedVinRef.current === cleanVin) return;
+    if (!isValidVin(cleanVin) || preDecodeRef.current?.vin === cleanVin) return;
 
-    const controller = new AbortController();
     const timer = window.setTimeout(() => {
-      preDecodedVinRef.current = cleanVin;
-      // Warm the backend decode chain; the result is intentionally discarded.
-      fetch("/api/vehicles/decode", {
-        method: "POST",
-        headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ vin: cleanVin }),
-        signal: controller.signal,
-      }).catch(() => {
-        /* speculative warm — ignore result/errors; the button-click decode owns the outcome */
-      });
+      const promise = api.post<any>("/vehicles/decode", { vin: cleanVin });
+      // Swallow here only to avoid an unhandled-rejection warning; the consumer
+      // (runDecode) attaches its own handler and surfaces the error to the user.
+      promise.catch(() => {});
+      preDecodeRef.current = { vin: cleanVin, promise };
     }, 350);
 
-    return () => {
-      window.clearTimeout(timer);
-      controller.abort();
-    };
+    return () => window.clearTimeout(timer);
   }, [vin]);
 
   // ─── Auto-focus ────────────────────────────────────────────────────────────
@@ -189,7 +181,24 @@ function SearchPage() {
     setLoading(true);
     const decodeStart = performance.now();
     try {
-      const data = await api.post<any>("/vehicles/decode", { vin: cleanVin });
+      // Reuse the speculative pre-decode when it's for this exact VIN (first
+      // attempt only) so the click doesn't fire a second identical request.
+      // Retries always decode fresh. If the reused warm failed, fall back once.
+      const pre = preDecodeRef.current;
+      const reuse = attempt === 1 && pre && pre.vin === cleanVin ? pre : null;
+      if (reuse) preDecodeRef.current = null;
+      let data: any;
+      try {
+        data = reuse
+          ? await reuse.promise
+          : await api.post<any>("/vehicles/decode", { vin: cleanVin });
+      } catch (reuseErr) {
+        if (reuse) {
+          data = await api.post<any>("/vehicles/decode", { vin: cleanVin });
+        } else {
+          throw reuseErr;
+        }
+      }
       const responseTimeMs = Math.round(performance.now() - decodeStart);
 
       if (data.candidates && Array.isArray(data.candidates)) {