Show real auth error on OAuth callback instead of generic message

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/views/OAuthCallbackView.vue

@@ -2,7 +2,7 @@
   <div class="min-h-screen bg-coffee-50 flex items-center justify-center">
     <div class="text-center">
       <div class="text-4xl mb-4">☕</div>
-      <p v-if="error" class="text-red-600">{{ error }}</p>
+      <p v-if="error || auth.error" class="text-red-600 max-w-sm px-4">{{ error || auth.error }}</p>
       <p v-else class="text-coffee-600">Signing you in…</p>
     </div>
   </div>
@@ -26,7 +26,7 @@ onMounted(async () => {
       await shops.fetchAll()
       router.replace('/')
     } else {
-      error.value = 'Authentication failed. Please try again.'
+      error.value = auth.error ?? 'Authentication failed — check console for details.'
     }
   } catch (e) {
     error.value = e instanceof Error ? e.message : 'Authentication error'