refactor(transcription): hoist @mention constants to shared module

Single source of truth for MAX_QUERY_LENGTH, SEARCH_DEBOUNCE_MS, and
SEARCH_RESULT_LIMIT — MentionDropdown imports MAX_QUERY_LENGTH;
PersonMentionEditor imports the debounce + result-limit; the spec's
mirror now imports SEARCH_DEBOUNCE_MS so it can never drift. Unblocks
the displayName length-cap fix (Felix #3 on PR #629).

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

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

@@ -4,17 +4,18 @@ import type { components } from '$lib/generated/api';
 import { formatLifeDateRange } from '$lib/person/personLifeDates';
 import { untrack } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
-
-type Person = components['schemas']['Person'];
-
 // Layered defence cap on the @mention search query length (CWE-400
 // amplification). The <input maxlength> attribute below caps direct
 // user edits, but the editor-mirror path (Tiptap contenteditable -> mirror
 // $effect -> searchQuery) is not covered by `maxlength` since the
 // contenteditable has no such enforcement. Clipping at the mirror keeps
 // the cap honest from both paths. Tracked server-side separately.
-// Nora #1 on PR #629.
-const MAX_QUERY_LENGTH = 100;
+// Nora #1 on PR #629. Hoisted to mentionConstants.ts so the host editor
+// (PersonMentionEditor) can clip the inserted displayName to the same cap
+// — see Felix #3 on PR #629.
+import { MAX_QUERY_LENGTH } from './mentionConstants';
+
+type Person = components['schemas']['Person'];
 
 // The dropdown receives a single reactive state object. PersonMentionEditor
 // mutates fields on this object (model.items = ..., etc.) and Svelte's $state