Strip orphan straight-quote pairings in JSX descriptions

The German „X" markers in the description prop combined a curly „
(U+201E) with a straight " (U+0022). The straight quote prematurely
terminated the JavaScript string inside the JSX expression. Removing
both markers around the example text keeps the description readable
and unambiguously valid JSX.

Test selector for the UnavailableWindow description updated to match
the new wording.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

studio-v2/app/stundenplan/_components/regeln/TeacherMaxHoursEditors.tsx

@@ -34,7 +34,7 @@ export function TeacherMaxHoursDayEditor() {
     <ConstraintShell
       testId="teacher-max-hours-day-editor"
       title="Lehrer: Max. Stunden / Tag"
-      description={"Beispiel: „Anna unterrichtet hoechstens 6 Stunden pro Tag" (oft Soft-Regel)."}
+      description={"Beispiel: Anna unterrichtet hoechstens 6 Stunden pro Tag (oft Soft-Regel)."}
       newLabel="+ Neue Regel"
       newDisabled={teachers.length === 0}
       prereqWarning={teachers.length === 0 ? 'Zuerst Lehrer anlegen.' : null}