Fix Playwright selector ambiguities in stundenplan.spec.ts

- 'Lehrer' label collided with a Sidebar entry; scope to <main nav>.
- 'Schmidt, Anna' lived in a closed <option>; assert on count via
  select.locator() instead of toBeVisible().

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

studio-v2/e2e/stundenplan.spec.ts

@@ -78,8 +78,10 @@ test.describe('Stundenplan — Page Shell', () => {
   })
 
   test('shows all 8 tabs', async ({ page }) => {
+    // Sidebar entries collide with tab labels for 'Lehrer' — scope to <nav> and use exact match.
+    const tabs = page.locator('main nav')
     for (const label of ['Klassen', 'Lehrer', 'Faecher', 'Raeume', 'Zeitraster', 'Stundentafel', 'Lehrauftraege', 'Regeln (Constraints)']) {
-      await expect(page.getByRole('button', { name: label })).toBeVisible()
+      await expect(tabs.getByRole('button', { name: label, exact: true })).toBeVisible()
     }
   })
 
@@ -203,8 +205,10 @@ test.describe('Stundenplan — Constraint editor empty teacher state', () => {
     const btn = page.getByRole('button', { name: '+ Neue Regel' })
     await expect(btn).toBeEnabled()
     await btn.click()
-    await expect(page.getByRole('combobox').first()).toBeVisible()
-    await expect(page.getByText('Schmidt, Anna')).toBeVisible()
+    const select = page.getByRole('combobox').first()
+    await expect(select).toBeVisible()
+    // <option> elements live inside <select>; assert on the option's text, not visibility.
+    await expect(select.locator('option', { hasText: 'Schmidt, Anna' })).toHaveCount(1)
   })
 })