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/PersonMentionEditor.svelte

@@ -43,6 +43,7 @@ let debounceTimer: ReturnType<typeof setTimeout> | undefined;
 
 function attachTextarea(node: HTMLTextAreaElement) {
 	textarea = node;
+	resizeTextarea();
 	const parentCleanup = captureTextarea?.(node);
 	return () => {
 		parentCleanup?.();
@@ -50,6 +51,19 @@ function attachTextarea(node: HTMLTextAreaElement) {
 	};
 }
 
+function resizeTextarea() {
+	if (!textarea) return;
+	textarea.style.height = 'auto';
+	textarea.style.height = `${textarea.scrollHeight}px`;
+}
+
+// Autoresize on every value change — read `value` so this $effect
+// re-runs whenever the bound prop is reassigned.
+$effect(() => {
+	void value;
+	resizeTextarea();
+});
+
 function handleInput() {
 	if (!textarea) return;
 	const cursorPos = textarea.selectionStart;