test(person-mention): boundary cases for whitespace + newline triggers

Tester #5506 nit pile:
- '@Aug @Bert' with cursor past the second @ — confirm the most
  recent @ wins (this is the canonical case for typing two mentions
  separated by a space).
- '@Aug\\nfoo' with cursor exactly at the newline (index 4) — the
  query still reads 'Aug' because the newline is past the cursor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/utils/personMention.spec.ts

@@ -42,6 +42,18 @@ describe('detectPersonMention', () => {
 		expect(detectPersonMention('@Aug@bar', 8)).toBeNull();
 	});
 
+	it('uses the most recent @ when separated by whitespace', () => {
+		// '@Aug @Bert' with cursor at end — the second @ is the trigger.
+		expect(detectPersonMention('@Aug @Bert', 10)).toBe('Bert');
+	});
+
+	it('returns the query when the cursor sits exactly at a newline boundary', () => {
+		// '@Aug\nfoo' with cursor at index 4 — right at the newline before it
+		// is consumed. The query is still 'Aug' because nothing past the cursor
+		// counts.
+		expect(detectPersonMention('@Aug\nfoo', 4)).toBe('Aug');
+	});
+
 	it('returns null when cursor is before the @', () => {
 		expect(detectPersonMention('@Hans', 0)).toBeNull();
 	});