refactor(transcription): rename @mention test-host to test-fixture

Test-only helper colocated with production code now has a visible
.test-fixture.svelte boundary so eslint-boundaries and code search
do not confuse it for a production component. The internal alias was
also bumped from *Host to *Fixture for consistency. No behaviour
change. Felix #3 / Nora #3 on PR #629.

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

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

@@ -2,7 +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 MentionDropdownFixture from './MentionDropdown.test-fixture.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import type { components } from '$lib/generated/api';
 
@@ -199,7 +199,7 @@ describe('MentionDropdown — search input', () => {
 
 	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, {
+		render(MentionDropdownFixture, {
 			model: baseModel(),
 			initialEditorQuery: 'WdG',
 			onReady: (s: (q: string) => void) => {