refactor(test): complete .test-host -> .test-fixture rename sweep

Round 2 renamed only MentionDropdown's fixture; three siblings retained
the old suffix. Rename PersonMentionEditor, confirm, and TranscriptionBlock
test hosts to the .test-fixture suffix and update the three importers so
the boundary is uniform across the repo. Felix #1 / Tobi #1 on PR #629.

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

frontend/src/lib/shared/discussion/PersonMentionEditor.test-fixture.svelte

@@ -0,0 +1,39 @@
+<script lang="ts">
+import { untrack } from 'svelte';
+import PersonMentionEditor from './PersonMentionEditor.svelte';
+import type { components } from '$lib/generated/api';
+
+type PersonMention = components['schemas']['PersonMention'];
+
+type Props = {
+	initialValue?: string;
+	initialMentions?: PersonMention[];
+	placeholder?: string;
+	disabled?: boolean;
+	onChange: (snapshot: { value: string; mentionedPersons: PersonMention[] }) => void;
+};
+
+let {
+	initialValue = '',
+	initialMentions = [],
+	placeholder,
+	disabled = false,
+	onChange
+}: Props = $props();
+
+// initial* props seed mount-time state; reading them inside untrack signals
+// the intentional one-shot capture and silences state_referenced_locally.
+let value = $state(untrack(() => initialValue));
+let mentionedPersons = $state<PersonMention[]>(untrack(() => [...initialMentions]));
+
+$effect(() => {
+	onChange({ value, mentionedPersons: [...mentionedPersons] });
+});
+</script>
+
+<PersonMentionEditor
+	bind:value={value}
+	bind:mentionedPersons={mentionedPersons}
+	placeholder={placeholder}
+	disabled={disabled}
+/>