fix(#248): show merge success banner via PRG pattern (?merged=1 redirect)

After a successful merge, redirect 303 to /admin/tags/{targetId}?merged=1.
Load function detects the param and returns mergeSuccess:true; +page.svelte
renders the banner and cleans the URL with replaceState so refresh doesn't
re-show it.

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

frontend/src/routes/admin/tags/[id]/+page.server.ts

@@ -3,11 +3,11 @@ import type { PageServerLoad, Actions } from './$types';
 import { createApiClient } from '$lib/api.server';
 import { getErrorMessage } from '$lib/errors';
 
-export const load: PageServerLoad = async ({ params, parent }) => {
+export const load: PageServerLoad = async ({ params, parent, url }) => {
 	const { tags } = await parent();
 	const tag = tags.find((t: { id: string }) => t.id === params.id);
 	if (!tag) throw error(404, getErrorMessage('TAG_NOT_FOUND'));
-	return { tag };
+	return { tag, mergeSuccess: url.searchParams.has('merged') };
 };
 
 export const actions: Actions = {
@@ -47,7 +47,7 @@ export const actions: Actions = {
 			return fail(result.response.status, { error: getErrorMessage(code) });
 		}
 
-		return { mergeSuccess: true, mergeTargetId: result.data!.id };
+		throw redirect(303, `/admin/tags/${result.data!.id}?merged=1`);
 	},
 
 	delete: async ({ params, request, fetch }) => {

frontend/src/routes/admin/tags/[id]/+page.svelte

@@ -1,5 +1,7 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
+import { replaceState } from '$app/navigation';
+import { page } from '$app/stores';
 import { m } from '$lib/paraglide/messages.js';
 import { createUnsavedWarning } from '$lib/hooks/useUnsavedWarning.svelte';
 import UnsavedWarningBanner from '$lib/components/UnsavedWarningBanner.svelte';
@@ -40,6 +42,12 @@ $effect(() => {
 	if (form?.success) unsaved.clearOnSuccess();
 });
 
+$effect(() => {
+	if (data.mergeSuccess) {
+		replaceState($page.url.pathname, {});
+	}
+});
+
 const colors = [
 	'sage',
 	'sienna',
@@ -86,6 +94,15 @@ const colors = [
 		{#if unsaved.showUnsavedWarning}
 			<UnsavedWarningBanner onDiscard={unsaved.discard} />
 		{/if}
+		{#if data.mergeSuccess}
+			<div
+				class="mb-5 rounded border border-green-200 bg-green-50 p-3 text-sm text-green-700"
+				role="status"
+				aria-live="polite"
+			>
+				{m.admin_tag_merge_success()}
+			</div>
+		{/if}
 		{#if form?.success}
 			<div class="mb-5 rounded border border-green-200 bg-green-50 p-3 text-sm text-green-700">
 				{m.admin_tag_updated()}

frontend/src/routes/admin/tags/[id]/TagMergeZone.svelte

@@ -1,5 +1,4 @@
 <script lang="ts">
-import { goto } from '$app/navigation';
 import { enhance } from '$app/forms';
 import { m } from '$lib/paraglide/messages.js';
 import TagParentPicker from '$lib/components/TagParentPicker.svelte';
@@ -15,7 +14,7 @@ type FlatTag = {
 interface Props {
 	tag: { id: string; name: string; documentCount: number };
 	allTags: FlatTag[];
-	form: { error?: string; mergeSuccess?: boolean; mergeTargetId?: string } | null;
+	form: { error?: string } | null;
 }
 
 let { tag, allTags, form }: Props = $props();
@@ -27,11 +26,6 @@ $effect(() => {
 	targetId = '';
 });
 
-$effect(() => {
-	if (form?.mergeSuccess && form.mergeTargetId) {
-		goto(`/admin/tags/${form.mergeTargetId}`);
-	}
-});
 const step = $derived(targetId ? 2 : 1);
 
 // All descendants of tag.id (to exclude from picker)
@@ -125,12 +119,6 @@ const targetTag = $derived(allTags.find((t) => t.id === targetId));
 		</div>
 	{/if}
 
-	{#if form?.mergeSuccess}
-		<p data-testid="merge-success-banner" class="mt-3 text-xs text-green-700">
-			{m.admin_tag_merge_success()}
-		</p>
-	{/if}
-
 	{#if form?.error}
 		<p class="mt-3 text-xs text-red-600">{form.error}</p>
 	{/if}

frontend/src/routes/admin/tags/[id]/TagMergeZone.svelte.spec.ts

@@ -4,7 +4,6 @@ import { page } from 'vitest/browser';
 import TagMergeZone from './TagMergeZone.svelte';
 
 vi.mock('$app/forms', () => ({ enhance: () => () => {} }));
-vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
 
 beforeEach(() => {
 	vi.useFakeTimers();
@@ -94,19 +93,3 @@ describe('TagMergeZone – stale state reset', () => {
 		expect(document.querySelector('[data-testid="merge-step2"]')).toBeFalsy();
 	});
 });
-
-describe('TagMergeZone – success banner', () => {
-	it('shows success banner when form.mergeSuccess is true', async () => {
-		render(TagMergeZone, {
-			tag,
-			allTags,
-			form: { mergeSuccess: true, mergeTargetId: 't2' }
-		});
-		await expect.element(page.getByTestId('merge-success-banner')).toBeInTheDocument();
-	});
-
-	it('does not show success banner when form is null', async () => {
-		const { container } = render(TagMergeZone, { tag, allTags, form: null });
-		expect(container.querySelector('[data-testid="merge-success-banner"]')).toBeFalsy();
-	});
-});

frontend/src/routes/admin/tags/[id]/page.server.spec.ts

@@ -1,5 +1,5 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
-import { actions } from './+page.server';
+import { actions, load } from './+page.server';
 
 const mockApi = {
 	PUT: vi.fn(),
@@ -13,6 +13,30 @@ vi.mock('$lib/api.server', () => ({
 
 beforeEach(() => vi.clearAllMocks());
 
+// ─── load function ─────────────────────────────────────────────────────────────
+
+describe('tags/[id] — load function', () => {
+	it('returns mergeSuccess: true when url has ?merged param', async () => {
+		const url = new URL('http://localhost/admin/tags/t1?merged=1');
+		const result = await load({
+			params: { id: 't1' },
+			parent: async () => ({ tags: [{ id: 't1', name: 'Test', documentCount: 0 }] }),
+			url
+		} as never);
+		expect((result as { mergeSuccess: boolean }).mergeSuccess).toBe(true);
+	});
+
+	it('returns mergeSuccess: false when url has no ?merged param', async () => {
+		const url = new URL('http://localhost/admin/tags/t1');
+		const result = await load({
+			params: { id: 't1' },
+			parent: async () => ({ tags: [{ id: 't1', name: 'Test', documentCount: 0 }] }),
+			url
+		} as never);
+		expect((result as { mergeSuccess: boolean }).mergeSuccess).toBe(false);
+	});
+});
+
 // ─── update action ─────────────────────────────────────────────────────────────
 
 describe('tags/[id] — update action', () => {
@@ -53,7 +77,7 @@ describe('tags/[id] — update action', () => {
 // ─── merge action ─────────────────────────────────────────────────────────────
 
 describe('tags/[id] — merge action', () => {
-	it('returns mergeSuccess and mergeTargetId on successful merge', async () => {
+	it('redirects to target tag with ?merged=1 on successful merge', async () => {
 		mockApi.POST.mockResolvedValue({
 			response: { ok: true },
 			data: { id: 't2', name: 'Reise' }
@@ -62,13 +86,19 @@ describe('tags/[id] — merge action', () => {
 		const formData = new FormData();
 		formData.set('targetId', 't2');
 
-		const result = await actions.merge({
-			params: { id: 't1' },
-			request: { formData: async () => formData },
-			fetch
-		} as never);
+		let redirectUrl: string | null = null;
+		try {
+			await actions.merge({
+				params: { id: 't1' },
+				request: { formData: async () => formData },
+				fetch
+			} as never);
+		} catch (e: unknown) {
+			const r = e as { location?: string };
+			redirectUrl = r.location ?? null;
+		}
 
-		expect(result).toEqual({ mergeSuccess: true, mergeTargetId: 't2' });
+		expect(redirectUrl).toBe('/admin/tags/t2?merged=1');
 	});
 
 	it('returns fail when merge API responds not ok', async () => {

frontend/src/routes/admin/tags/[id]/page.svelte.spec.ts

@@ -4,7 +4,19 @@ import { page } from 'vitest/browser';
 import Page from './+page.svelte';
 
 vi.mock('$app/forms', () => ({ enhance: () => () => {} }));
-vi.mock('$app/navigation', () => ({ beforeNavigate: vi.fn(), goto: vi.fn() }));
+vi.mock('$app/navigation', () => ({
+	beforeNavigate: vi.fn(),
+	goto: vi.fn(),
+	replaceState: vi.fn()
+}));
+vi.mock('$app/stores', () => ({
+	page: {
+		subscribe: (fn: (v: { url: URL }) => void) => {
+			fn({ url: new URL('http://localhost/admin/tags/t1') });
+			return () => {};
+		}
+	}
+}));
 
 import { beforeNavigate, goto } from '$app/navigation';
 
@@ -138,6 +150,20 @@ describe('Admin edit tag page – color picker', () => {
 	});
 });
 
+// ─── Merge success banner ─────────────────────────────────────────────────────
+
+describe('Admin edit tag page – merge success banner', () => {
+	it('shows merge success banner when data.mergeSuccess is true', async () => {
+		render(Page, { data: { ...baseData, mergeSuccess: true }, form: null });
+		await expect.element(page.getByText(/Erfolgreich zusammengeführt/i)).toBeInTheDocument();
+	});
+
+	it('does not show merge success banner when data.mergeSuccess is false', async () => {
+		render(Page, { data: { ...baseData, mergeSuccess: false }, form: null });
+		await expect.element(page.getByText(/Erfolgreich zusammengeführt/i)).not.toBeInTheDocument();
+	});
+});
+
 // ─── New components present ───────────────────────────────────────────────────
 
 describe('Admin edit tag page – new components', () => {