fix(rtl): write the isolate marks of flowText as escapes, not as raw characters

A raw FSI or PDI in the source reads as nothing and is what Sonar's
bidirectional character rule (S6389) flags. The strings are the same.

client/src/utils/flowText.test.ts

@@ -7,8 +7,8 @@ import { afterEach, describe, expect, it } from 'vitest';
 
 import { flowText, isRtlDocument } from './flowText';
 
-const FSI = '⁨';
-const PDI = '⁩';
+const FSI = '\u2068';
+const PDI = '\u2069';
 
 describe('flowText', () => {
   afterEach(() => {

client/src/utils/flowText.ts

@@ -9,8 +9,8 @@
  * script the names are in, and the arrow points left, from the first name to
  * the second. A left-to-right UI gets the plain string.
  */
-const FSI = '⁨';
-const PDI = '⁩';
+const FSI = '\u2068';
+const PDI = '\u2069';
 
 export function isRtlDocument(): boolean {
   return typeof document !== 'undefined' && document.documentElement.dir === 'rtl';