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/api/client.ts

@@ -543,12 +543,15 @@ export interface SetupStatusResponse {
  * Payload for POST /api/setup/config.
  * Collects non-secret runtime config written to the app_config table (D-02).
  * No secrets — VAPID private key and encryption key stay in Docker env.
+ *
+ * Field names match the API's configSchema exactly (camelCase).
+ * API contract: { oidcIssuer, oidcClientId, vapidPublicKey, appExternalUrl }
  */
 export interface SetupConfigPayload {
-  app_url: string;
-  oidc_issuer: string;
-  oidc_client_id: string;
-  vapid_public_key: string;
+  appExternalUrl: string;
+  oidcIssuer: string;
+  oidcClientId: string;
+  vapidPublicKey: string;
 }
 
 /**
@@ -594,8 +597,26 @@ export async function postSetupConfig(payload: SetupConfigPayload): Promise<void
   }
 
   if (!res.ok) {
-    const body = (await res.json().catch(() => ({}))) as { error?: string };
-    throw new Error(body.error ?? `POST /api/setup/config failed: ${res.status}`);
+    const body = (await res.json().catch(() => ({}))) as {
+      error?: string | { name?: string; issues?: Array<{ message: string }> };
+    };
+    // body.error may be a ZodError object ({ name: "ZodError", issues: [...] })
+    // rather than a plain string — extract the first issue message to avoid
+    // "[object Object]" appearing in the UI (BUG 2 fix).
+    let message: string;
+    if (typeof body.error === 'string') {
+      message = body.error;
+    } else if (
+      body.error &&
+      typeof body.error === 'object' &&
+      Array.isArray((body.error as { issues?: unknown[] }).issues) &&
+      (body.error as { issues: Array<{ message: string }> }).issues.length > 0
+    ) {
+      message = (body.error as { issues: Array<{ message: string }> }).issues[0].message;
+    } else {
+      message = `POST /api/setup/config failed: ${res.status}`;
+    }
+    throw new Error(message);
   }
 }