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.test-host.svelte

@@ -9,10 +9,17 @@ type Props = {
 	initialValue?: string;
 	initialMentions?: PersonMention[];
 	placeholder?: string;
+	disabled?: boolean;
 	onChange: (snapshot: { value: string; mentionedPersons: PersonMention[] }) => void;
 };
 
-let { initialValue = '', initialMentions = [], placeholder, onChange }: Props = $props();
+let {
+	initialValue = '',
+	initialMentions = [],
+	placeholder,
+	disabled = false,
+	onChange
+}: Props = $props();
 
 // initial* props seed mount-time state; reading them inside untrack signals
 // the intentional one-shot capture and silences state_referenced_locally.
@@ -28,4 +35,5 @@ $effect(() => {
 	bind:value={value}
 	bind:mentionedPersons={mentionedPersons}
 	placeholder={placeholder}
+	disabled={disabled}
 />