feat(web): add error/not-found state + safe back nav to vehicle page

The vehicle detail page had no error handling: a failed /vehicles/:id (404 or
network) rendered a broken header ("  ()") with no retry, while only the
categories sub-page had a proper error block. Now a failed load shows a
not-found vs. generic-error alert with retry (mirroring the category page), and
404s skip retries so the state appears immediately. The back button now uses
the router history and falls back to /dashboard/search on deep links instead of
window.history.back() leaving the site.

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

apps/web/src/routes/dashboard/vehicles_/$id/index.tsx

@@ -3,13 +3,13 @@ import { CategoryGrid } from "@/components/categories/category-grid";
 import { CategoryTree } from "@/components/categories/category-tree";
 import { CategoryViewToggle } from "@/components/categories/category-view-toggle";
 import { CarBrandLogo } from "@/components/ui/car-brand-logo";
-import { api } from "@/lib/api-client";
+import { ApiError, api } from "@/lib/api-client";
 import { getUserSettings, setUserSetting } from "@/lib/user-settings";
 import { Card, CardContent, CardHeader, CardTitle } from "@sase/ui";
 import { Skeleton } from "@sase/ui";
 import { Button } from "@sase/ui";
 import { useQuery } from "@tanstack/react-query";
-import { createFileRoute } from "@tanstack/react-router";
+import { createFileRoute, useCanGoBack, useNavigate, useRouter } from "@tanstack/react-router";
 import { ArrowLeft } from "lucide-react";
 import { useState } from "react";
 
@@ -29,10 +29,31 @@ function VehicleDetailPage() {
     setUserSetting("categoryViewMode", mode);
   };
 
-  const { data: vehicle, isLoading: vehicleLoading } = useQuery({
+  const router = useRouter();
+  const canGoBack = useCanGoBack();
+  const navigate = useNavigate();
+
+  // Go back within the app; if the page was opened from a deep link (no app
+  // history), fall back to search instead of leaving the site.
+  const handleBack = () => {
+    if (canGoBack) router.history.back();
+    else navigate({ to: "/dashboard/search" });
+  };
+
+  const {
+    data: vehicle,
+    isLoading: vehicleLoading,
+    isError: vehicleError,
+    error: vehicleErrorObj,
+    refetch: refetchVehicle,
+    isFetching: vehicleFetching,
+  } = useQuery({
     queryKey: ["vehicle", id],
     queryFn: () => api.get<any>(`/vehicles/${id}`),
     enabled: !!id,
+    // A missing vehicle (404) is final — don't burn retries before showing the
+    // not-found state. Transient errors still retry.
+    retry: (count, err) => !(err instanceof ApiError && err.status === 404) && count < 2,
   });
 
   const { data: categoryTree, isLoading: categoriesLoading } = useQuery({
@@ -78,6 +99,60 @@ function VehicleDetailPage() {
     );
   }
 
+  if (vehicleError) {
+    const is404 = vehicleErrorObj instanceof ApiError && vehicleErrorObj.status === 404;
+    return (
+      <div className="space-y-6">
+        <div className="flex items-center gap-3">
+          <Button
+            variant="ghost"
+            size="icon"
+            onClick={handleBack}
+            title="Geri dön"
+            data-faro-user-action-name="vehicle-back"
+          >
+            <ArrowLeft className="h-4 w-4" />
+          </Button>
+          <h2 className="text-xl font-bold">{is404 ? "Araç bulunamadı" : "Araç yüklenemedi"}</h2>
+        </div>
+        <div
+          role="alert"
+          className="flex flex-col items-start gap-3 rounded-lg border border-destructive/40 bg-destructive/5 p-5 text-sm"
+        >
+          <div>
+            <p className="font-medium text-destructive">
+              {is404 ? "Bu araç kaydı bulunamadı" : "Araç bilgileri yüklenemedi"}
+            </p>
+            <p className="mt-1 text-muted-foreground">
+              {is404
+                ? "Bağlantı geçersiz olabilir ya da kayıt kaldırılmış olabilir."
+                : vehicleErrorObj instanceof Error
+                  ? vehicleErrorObj.message
+                  : "Veriler yüklenirken bir hata oluştu."}
+            </p>
+          </div>
+          <div className="flex flex-wrap gap-2">
+            {!is404 && (
+              <Button
+                type="button"
+                size="sm"
+                variant="outline"
+                onClick={() => refetchVehicle()}
+                disabled={vehicleFetching}
+                data-faro-user-action-name="vehicle-retry"
+              >
+                {vehicleFetching ? "Yükleniyor…" : "Tekrar dene"}
+              </Button>
+            )}
+            <Button type="button" size="sm" variant="ghost" onClick={handleBack}>
+              Geri dön
+            </Button>
+          </div>
+        </div>
+      </div>
+    );
+  }
+
   return (
     <div className="space-y-6">
       {/* Header */}
@@ -85,7 +160,7 @@ function VehicleDetailPage() {
         <Button
           variant="ghost"
           size="icon"
-          onClick={() => window.history.back()}
+          onClick={handleBack}
           title="Geri dön"
           data-faro-user-action-name="vehicle-back"
         >