test(PersonMentionEditor): assert no HTML injection via mention displayName

Adds a CWE-79 regression test: a sidecar entry whose displayName contains
an <img onerror=alert(1)> payload must round-trip through deserialize and
the Tiptap renderHTML without producing a real <img> element in the editor
DOM. Locks down the "renderHTML's third tuple entry is a text node, never
parsed as HTML" invariant so a future "use innerHTML for performance"
refactor cannot silently regress.

Nora #5618 detection-gap concern.

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

frontend/src/lib/components/PersonMentionEditor.svelte.spec.ts

@@ -316,6 +316,40 @@ describe('PersonMentionEditor — disabled state', () => {
 	});
 });
 
+// ─── Security — XSS in displayName (CWE-79) ──────────────────────────────────
+
+describe('PersonMentionEditor — XSS resistance', () => {
+	it('renders a malicious displayName as text, not as HTML elements', async () => {
+		// A historical sidecar entry whose displayName contains an HTML payload
+		// that would execute if interpolated as raw HTML. Tiptap's renderHTML
+		// returns the @-prefixed string as the third tuple entry, which
+		// ProseMirror's DOMSerializer treats as a Text node — escaping it.
+		const maliciousMention: PersonMention = {
+			personId: '00000000-0000-0000-0000-000000000001',
+			displayName: '<img src=x onerror=alert(1)>'
+		};
+
+		renderHost({
+			value: '@<img src=x onerror=alert(1)>',
+			mentionedPersons: [maliciousMention]
+		});
+
+		await vi.waitFor(() => {
+			const textbox = document.querySelector('[role="textbox"]') as HTMLElement | null;
+			expect(textbox).not.toBeNull();
+			// No element from the malicious payload should have appeared as a real
+			// DOM node. (Tiptap inserts its own ProseMirror-separator <img> in empty
+			// paragraphs — that is internal markup and never carries user attrs;
+			// guard against the injection by checking the user-controlled attrs.)
+			expect(textbox!.querySelector('img[onerror]')).toBeNull();
+			expect(textbox!.querySelector('img[src="x"]')).toBeNull();
+			expect(textbox!.querySelector('script')).toBeNull();
+			// The payload should appear as visible text content instead.
+			expect(textbox!.textContent ?? '').toContain('<img src=x onerror=alert(1)>');
+		});
+	});
+});
+
 // ─── Touch target (WCAG 2.2 AA) ──────────────────────────────────────────────
 
 describe('PersonMentionEditor — touch target', () => {