refactor(person-mention): centralise PERSON_MENTION_SELECTOR constant

Markus flagged that 'a.person-mention' is a magic string repeated four times
in TranscriptionReadView, plus the CSS rule, plus tests. Extract into a single
exported constant so the renderer template, the delegated event handlers,
and the consumer-side selectors all import the same value.

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

frontend/src/lib/utils/mention.ts

@@ -1,5 +1,15 @@
 import type { MentionDTO, PersonMention } from '$lib/types';
 
+/**
+ * Single-source CSS selector for rendered person-mention anchors. Used by:
+ *   - layout.css            (.person-mention rule, focus ring, underline)
+ *   - TranscriptionReadView (delegated mouseenter/leave/click handlers)
+ *   - unit + e2e tests
+ *
+ * Keep these in sync — the renderer template below emits exactly this class.
+ */
+export const PERSON_MENTION_SELECTOR = 'a.person-mention';
+
 /**
  * Branded string type for HTML that has been pre-escaped and assembled by
  * one of the trusted renderers in this module. The brand exists so that