test(transcription): pin sticky search-input takeover behaviour

Once the user edits the dropdown search input, subsequent editorQuery
changes from the host editor must not overwrite it. Felix on PR #629.
Adds a small test host that exposes a setter for editorQuery so the
test can drive reactive prop changes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
 import MentionDropdown from './MentionDropdown.svelte';
+import MentionDropdownHost from './MentionDropdown.test-host.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import type { components } from '$lib/generated/api';
 
@@ -195,4 +196,25 @@ describe('MentionDropdown — search input', () => {
 			expect(onSearch).toHaveBeenLastCalledWith('Walter');
 		});
 	});
+
+	it('keeps the user-edited search value when editorQuery changes after the takeover (Felix on PR #629)', async () => {
+		let setEditorQuery!: (q: string) => void;
+		render(MentionDropdownHost, {
+			model: baseModel(),
+			initialEditorQuery: 'WdG',
+			onReady: (s: (q: string) => void) => {
+				setEditorQuery = s;
+			}
+		});
+
+		await expect.element(page.getByRole('searchbox')).toHaveValue('WdG');
+
+		await page.getByRole('searchbox').fill('Walter');
+		await expect.element(page.getByRole('searchbox')).toHaveValue('Walter');
+
+		setEditorQuery('WdGruyter');
+		await new Promise((r) => setTimeout(r, 50));
+
+		await expect.element(page.getByRole('searchbox')).toHaveValue('Walter');
+	});
 });