refactor(person-mention): rename shadowed m param in TranscriptionBlock bind setter

Same fix as 79349644 — the bind:mentionedPersons setter parameter `m`
shadowed the imported Paraglide m helper used two lines later in
placeholder={m.transcription_block_placeholder()}. Functionally fine
because the inner scope ends before the outer reference, but a clarity
trap. Renamed to next.

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -198,8 +198,8 @@ function handleTextareaMouseUp() {
 						emitChange();
 					}}
 				bind:mentionedPersons={() => localMentions,
-					(m) => {
-						localMentions = m;
+					(next) => {
+						localMentions = next;
 						emitChange();
 					}}
 				placeholder={m.transcription_block_placeholder()}