feat(web): speculative pre-decode on a valid VIN (faster counter lookups)

The moment the VIN reaches 17 valid chars, warm the full decode in the background
(350ms debounce, deduped per VIN, fire-and-forget) so the "Şase Çöz" click hits
the backend lock / 24h positive cache and returns near-instantly. If the warm is
still in flight when the dealer clicks, the button's decode lock-waits on it and
navigates the moment it lands.

No analytics/state/navigation from the pre-decode — vin_decoded and the
success/candidates/error events still fire only from the button path, so the CRO
funnel stays = real intent. Raw fetch + AbortController (api.post takes no signal),
mirroring the existing 17-char preview effect.

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

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

@@ -61,6 +61,7 @@ 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);
   const [vin, setVin] = useState("");
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -147,6 +148,36 @@ function SearchPage() {
     return () => controller.abort();
   }, [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.
+  useEffect(() => {
+    const cleanVin = vin.toUpperCase().trim();
+    if (!isValidVin(cleanVin) || preDecodedVinRef.current === 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 */
+      });
+    }, 350);
+
+    return () => {
+      window.clearTimeout(timer);
+      controller.abort();
+    };
+  }, [vin]);
+
   // ─── Auto-focus ────────────────────────────────────────────────────────────
   useEffect(() => {
     inputRef.current?.focus();