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/e2e/stundenplan.spec.ts

@@ -312,7 +312,7 @@ test.describe('Stundenplan — Regeln Hub', () => {
   test('switching to UnavailableWindow editor swaps the right pane', async ({ page }) => {
     await page.getByRole('button', { name: 'Zeitfenster nicht verfuegbar' }).click()
     await expect(page.getByTestId('teacher-unavailable-window-editor')).toBeVisible()
-    await expect(page.getByText('„Lehrer Z Dienstags 13:00–17:00 nicht".')).toBeVisible()
+    await expect(page.getByText('Lehrer Z Dienstags 13:00–17:00 nicht.')).toBeVisible()
   })
 
   test('switching to SubjectMaxConsecutive editor works', async ({ page }) => {