test(mocks): add shared $app/forms interceptor mock body

Single home for the non-trivial form-interceptor enhance() shared by the
four complex consumers: it intercepts submit, invokes the SubmitFunction,
and fires the returned callback with a configurable result. setFormResult()
drives the success/failure branch; an embedded beforeEach resets it before
every test so isolation is structural. Consumed via vi.mock('$app/forms',
() => ({ ...formsMock })) through the $mocks alias. Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/__mocks__/$app/forms.ts

@@ -0,0 +1,43 @@
+// Shared browser-test mock body for the SvelteKit `$app/forms` virtual module.
+//
+// Imported into a sync vi.mock factory via the $mocks alias:
+//   import * as formsMock from '$mocks/$app/forms';
+//   vi.mock('$app/forms', () => ({ ...formsMock }));
+//
+// `enhance` intercepts the form's submit event, invokes the component's
+// SubmitFunction, and — when that returns a post-submit callback — calls it
+// with the configurable `_formResult`. Tests drive the success/failure branch
+// with `setFormResult({ type: 'failure' })`. The embedded `beforeEach` resets
+// the result before every test, so isolation is structural, not per-spec.
+// See ADR-012.
+
+import { beforeEach } from 'vitest';
+
+export type FormEnhanceResult = { type: string; data?: Record<string, unknown> };
+
+let _formResult: FormEnhanceResult = { type: 'success' };
+
+export function setFormResult(result: FormEnhanceResult): void {
+	_formResult = result;
+}
+
+export function enhance(
+	node: HTMLFormElement,
+	submit?: (opts: {
+		formData: FormData;
+	}) => (opts: { result: FormEnhanceResult; update: () => Promise<void> }) => Promise<void>
+): { destroy: () => void } {
+	const handler = async (e: Event) => {
+		e.preventDefault();
+		const callback = submit?.({ formData: new FormData(node) } as never);
+		if (typeof callback === 'function') {
+			await callback({ result: _formResult, update: async () => {} });
+		}
+	};
+	node.addEventListener('submit', handler);
+	return { destroy: () => node.removeEventListener('submit', handler) };
+}
+
+beforeEach(() => {
+	_formResult = { type: 'success' };
+});