fix: pin black to 26.3.1 across all environments

- Pin black version in requirements-dev.txt (was >=24.0.0)
- Update pre-commit to use black 26.3.1 with Python 3.12
- Add language_version: python3.12 to pre-commit black hook
- Reformat files with new black version for consistency

tests/test_overview_ltv_history_playwright.py

@@ -26,16 +26,14 @@ def test_overview_shows_ltv_history_and_exports_csv() -> None:
         expect(page.locator("text=90 Day").first).to_be_visible(timeout=15000)
         expect(page.get_by_role("button", name="Export CSV")).to_be_visible(timeout=15000)
 
-        series_names = page.evaluate(
-            """
+        series_names = page.evaluate("""
             async () => {
               const importMap = JSON.parse(document.querySelector('script[type="importmap"]').textContent).imports;
               const mod = await import(importMap['nicegui-echart']);
               const chart = mod.echarts.getInstanceByDom(document.querySelector('.nicegui-echart'));
               return chart ? chart.getOption().series.map(series => series.name) : [];
             }
-            """
-        )
+            """)
         assert series_names == ["LTV", "Margin threshold"]
 
         with page.expect_download() as download_info: