fix(shared): null-harden settled() against placeholder slots

A Promise.resolve(null) placeholder (e.g. the gated drafts slot) fulfils
with a null value; settled() dereferenced v.response unconditionally and
threw. Now any nullish value resolves to null. Adds unit tests for all
settled() branches.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/shared/server/settled.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from 'vitest';
+import { settled } from './settled';
+
+describe('settled', () => {
+	it('returns the data for a fulfilled ok response', () => {
+		const res: PromiseSettledResult<unknown> = {
+			status: 'fulfilled',
+			value: { response: { ok: true } as Response, data: [{ id: '1' }] }
+		};
+		expect(settled<{ id: string }[]>(res)).toEqual([{ id: '1' }]);
+	});
+
+	it('returns null for a fulfilled non-ok response', () => {
+		const res: PromiseSettledResult<unknown> = {
+			status: 'fulfilled',
+			value: { response: { ok: false, status: 403 } as Response, data: undefined }
+		};
+		expect(settled(res)).toBeNull();
+	});
+
+	it('returns null for a rejected result', () => {
+		const res: PromiseSettledResult<unknown> = {
+			status: 'rejected',
+			reason: new Error('network error')
+		};
+		expect(settled(res)).toBeNull();
+	});
+
+	it('returns null for undefined input', () => {
+		expect(settled(undefined)).toBeNull();
+	});
+
+	it('returns null for a fulfilled null value (Promise.resolve(null) placeholder slot)', () => {
+		const res: PromiseSettledResult<unknown> = { status: 'fulfilled', value: null };
+		expect(settled(res)).toBeNull();
+	});
+});

frontend/src/lib/shared/server/settled.ts

@@ -1,5 +1,5 @@
 export function settled<T>(res: PromiseSettledResult<unknown> | undefined): T | null {
 	if (res?.status !== 'fulfilled') return null;
-	const v = res.value as { response: Response; data: unknown };
-	return v.response.ok ? ((v.data as T) ?? null) : null;
+	const v = res.value as { response: Response; data: unknown } | null;
+	return v?.response?.ok ? ((v.data as T) ?? null) : null;
 }