refactor(person-mention): rename shadowed Paraglide m variable in dedup check

Felix #1: inside selectPerson the .some((m) => ...) parameter shadowed the
imported Paraglide m helper. Functionally fine, but a footgun. Rename to
existing for clarity.

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

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -103,7 +103,7 @@ async function selectPerson(person: Person) {
 	const after = value.slice(cursorPos);
 	value = before + replacement + after;
 
-	if (!mentionedPersons.some((m) => m.personId === person.id)) {
+	if (!mentionedPersons.some((existing) => existing.personId === person.id)) {
 		mentionedPersons = [...mentionedPersons, { personId: person.id!, displayName }];
 	}