fix(PersonMentionEditor): guard setEditable effect against re-entry loop

The disabled-state effect calls editor.setEditable, which triggers a
ProseMirror transaction → onUpdate → bind:value/mentionedPersons writes →
host re-render → child prop pass-through → effect re-fires. Without an
idempotence check, this exceeds Svelte's effect_update_depth and crashes
every consuming spec (TranscriptionBlock 22/22). Compare editor.isEditable
against the desired value first; only call setEditable when it actually
needs to change.

Follow-up to 6ef888a1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -241,8 +241,15 @@ onDestroy(() => {
 // Keep editor in sync with the reactive `disabled` prop. Tiptap's setEditable
 // flips contenteditable on the inner DOM and stops accepting input — matches
 // the textarea's old `disabled` semantics for keyboard users (WCAG 2.1.1).
+//
+// Guard: setEditable triggers a ProseMirror transaction which fires onUpdate;
+// onUpdate writes through bind:value / bind:mentionedPersons. Without this
+// idempotence check, the effect would loop on every prop pass-through.
 $effect(() => {
-	editor?.setEditable(!disabled);
+	const shouldBeEditable = !disabled;
+	if (editor && editor.isEditable !== shouldBeEditable) {
+		editor.setEditable(shouldBeEditable);
+	}
 });
 </script>