fix(12-04): align setup config client to camelCase API contract + readable errors

BUG 1: Rename SetupConfigPayload fields from snake_case to camelCase to match
the API configSchema (appExternalUrl, oidcIssuer, oidcClientId, vapidPublicKey).
Update SetupPage.tsx handleSaveAndValidate to send the correct camelCase keys.

BUG 2: Extract human-readable message from ZodError object in postSetupConfig
error handler. When body.error is an object with issues[], use issues[0].message
instead of stringifying the object (which produces "[object Object]").

All 245 PWA tests pass; TypeScript clean.

apps/pwa/src/routes/SetupPage.tsx

@@ -502,10 +502,10 @@ function Step2Config({ onBack, onSuccess, stepHeadingRef }: Step2Props) {
     }
 
     configMutation.mutate({
-      app_url: appUrl.trim(),
-      oidc_issuer: oidcIssuer.trim(),
-      oidc_client_id: oidcClientId.trim(),
-      vapid_public_key: vapidPublicKey.trim(),
+      appExternalUrl: appUrl.trim(),
+      oidcIssuer: oidcIssuer.trim(),
+      oidcClientId: oidcClientId.trim(),
+      vapidPublicKey: vapidPublicKey.trim(),
     });
   }