fix(i18n): fall back to Accept-Language and add a language switcher to onboarding

New visitors always got the English default because locale resolution
only ever read the NEXT_LOCALE cookie — never set until a user visits
a page with the LanguageSwitcher, which only lives in the authenticated
app Sidebar. /onboarding/pet runs before that, so a German browser
landed in English with no way to change it (reported live after
deploying the upload-consent step from Gitea #3).

Adds an Accept-Language fallback in src/i18n/request.ts (only consulted
when no cookie is set) and renders the existing LanguageSwitcher on the
onboarding page so users can override the guess.

src/app/onboarding/pet/page.tsx

@@ -9,6 +9,7 @@ import { Button } from "@/components/ui/button";
 import { useActivePet } from "@/context/ActivePetContext";
 import { OnboardingGuidelines } from "@/components/legal/OnboardingGuidelines";
 import { UploadConsent } from "@/components/legal/UploadConsent";
+import { LanguageSwitcher } from "@/components/i18n/LanguageSwitcher";
 
 /**
  * Onboarding pet page — /onboarding/pet
@@ -32,55 +33,41 @@ export default function OnboardingPetPage() {
   const [guidelinesAcknowledged, setGuidelinesAcknowledged] = useState(false);
   const [uploadConsentGiven, setUploadConsentGiven] = useState(false);
 
+  let content: React.ReactNode;
+
   if (!guidelinesAcknowledged) {
-    return (
-      <main className="min-h-screen flex items-center justify-center bg-background px-4">
-        <OnboardingGuidelines onContinue={() => setGuidelinesAcknowledged(true)} />
-      </main>
-    );
-  }
-
-  if (!uploadConsentGiven) {
-    return (
-      <main className="min-h-screen flex items-center justify-center bg-background px-4">
-        <UploadConsent onContinue={() => setUploadConsentGiven(true)} />
-      </main>
-    );
-  }
-
-  if (createdPetId) {
-    return (
-      <main className="min-h-screen flex items-center justify-center bg-background px-4">
-        <div className="w-full max-w-[512px] p-8 flex flex-col items-center gap-6">
-          <div className="text-center">
-            <h1 className="text-xl font-semibold mb-2">{t("addPhoto")}</h1>
-            <p className="text-sm text-muted-foreground">{t("addPhotoDesc")}</p>
-          </div>
-
-          <AvatarUpload petId={createdPetId} onUploadComplete={() => {}} />
-
-          <div className="flex flex-col gap-2 w-full">
-            <Button
-              className="w-full"
-              onClick={() => router.push(`/pets/${createdPetId}`)}
-            >
-              {t("done")}
-            </Button>
-            <Button
-              variant="ghost"
-              className="w-full text-muted-foreground"
-              onClick={() => router.push(`/pets/${createdPetId}`)}
-            >
-              {t("skipForNow")}
-            </Button>
-          </div>
+    content = <OnboardingGuidelines onContinue={() => setGuidelinesAcknowledged(true)} />;
+  } else if (!uploadConsentGiven) {
+    content = <UploadConsent onContinue={() => setUploadConsentGiven(true)} />;
+  } else if (createdPetId) {
+    content = (
+      <div className="w-full max-w-[512px] p-8 flex flex-col items-center gap-6">
+        <div className="text-center">
+          <h1 className="text-xl font-semibold mb-2">{t("addPhoto")}</h1>
+          <p className="text-sm text-muted-foreground">{t("addPhotoDesc")}</p>
         </div>
-      </main>
-    );
-  }
 
-  return (
-    <main className="min-h-screen flex items-center justify-center bg-background px-4">
+        <AvatarUpload petId={createdPetId} onUploadComplete={() => {}} />
+
+        <div className="flex flex-col gap-2 w-full">
+          <Button
+            className="w-full"
+            onClick={() => router.push(`/pets/${createdPetId}`)}
+          >
+            {t("done")}
+          </Button>
+          <Button
+            variant="ghost"
+            className="w-full text-muted-foreground"
+            onClick={() => router.push(`/pets/${createdPetId}`)}
+          >
+            {t("skipForNow")}
+          </Button>
+        </div>
+      </div>
+    );
+  } else {
+    content = (
       <div className="w-full max-w-[512px] p-8">
         <h1 className="text-xl font-semibold mb-2">{t("addFirstPet")}</h1>
         <p className="text-sm text-muted-foreground mb-8">{t("addFirstPetDesc")}</p>
@@ -92,6 +79,15 @@ export default function OnboardingPetPage() {
           }}
         />
       </div>
+    );
+  }
+
+  return (
+    <main className="relative min-h-screen flex items-center justify-center bg-background px-4">
+      <div className="absolute top-4 right-4">
+        <LanguageSwitcher />
+      </div>
+      {content}
     </main>
   );
 }