fix(PersonMentionEditor): enforce disabled state on the contenteditable

Wrapping the editor with pointer-events-none was visual-only — keyboard users
could still tab into the contenteditable and type. Wire `editable: !disabled`
on the Tiptap Editor and a reactive `$effect` that calls setEditable when the
prop flips after mount; expose `aria-disabled="true"` on the wrapper so
screen readers announce the deactivated state.

Tests assert contenteditable=false and aria-disabled=true when disabled;
contenteditable=true otherwise.

Closes WCAG 2.1.1 / 4.1.2 — Felix #5615 + Leonie #5621 + Nora #5618 BLOCKER.

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

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -82,6 +82,11 @@ onMount(() => {
 
 	editor = new Editor({
 		element: editorEl,
+		// Initial editable state honors the `disabled` prop. The reactive
+		// $effect below keeps it in sync if the prop flips after mount —
+		// without this, a keyboard user can tab into the contenteditable
+		// even when the wrapper has pointer-events-none (WCAG 2.1.1).
+		editable: !disabled,
 		extensions: [
 			StarterKit.configure({
 				heading: false,
@@ -232,11 +237,19 @@ onMount(() => {
 onDestroy(() => {
 	editor?.destroy();
 });
+
+// Keep editor in sync with the reactive `disabled` prop. Tiptap's setEditable
+// flips contenteditable on the inner DOM and stops accepting input — matches
+// the textarea's old `disabled` semantics for keyboard users (WCAG 2.1.1).
+$effect(() => {
+	editor?.setEditable(!disabled);
+});
 </script>
 
 <div
 	class="relative rounded-sm border border-transparent focus-within:border-brand-mint focus-within:ring-2 focus-within:ring-brand-mint/40"
 	class:opacity-50={disabled}
 	class:pointer-events-none={disabled}
+	aria-disabled={disabled ? 'true' : undefined}
 	bind:this={editorEl}
 ></div>