refactor(person-mention): move autoresize into PersonMentionEditor

Felix #5: TranscriptionBlock had a `\$effect(() => { void localText; ... })`
hack to re-trigger autoresize on text change, plus a captureTextarea
callback that the parent only used to size a node it didn't own.

The editor owns the textarea — it should also size it. Move the
autoresize \$effect into PersonMentionEditor so the parent only
captures the node when it genuinely needs to read selection bounds
(quote selection still works).

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -85,24 +85,11 @@ let textareaEl: HTMLTextAreaElement | null = null;
 
 function captureTextarea(node: HTMLTextAreaElement) {
 	textareaEl = node;
-	resizeTextarea();
 	return () => {
 		textareaEl = null;
 	};
 }
 
-function resizeTextarea() {
-	if (!textareaEl) return;
-	textareaEl.style.height = 'auto';
-	textareaEl.style.height = `${textareaEl.scrollHeight}px`;
-}
-
-$effect(() => {
-	// Re-run autoresize whenever the bound text changes.
-	void localText;
-	resizeTextarea();
-});
-
 function emitChange() {
 	onTextChange(localText, localMentions);
 }