tests: the ECMWF fixture's track starts at +3H

The axis under test skips the analysis frame, which is the case the
start label exists for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016rq4r7RkWVVYhSu45xQWTD

Author: ringsaturn

tests/e2e/app.spec.ts

@@ -564,7 +564,7 @@ test("switching to ECMWF loads its own run on a mixed-cadence 240-hour timeline"
   const slider = page.getByRole("slider", { name: "Forecast hour" });
   await expect(slider).toHaveAttribute("max", "63");
   await expect(page.locator("#forecast-hour")).toHaveText("F003");
-  await expect(page.locator("#track-start")).toHaveText("+0H");
+  await expect(page.locator("#track-start")).toHaveText("+3H");
   await expect(page.locator("#track-horizon")).toHaveText("+240H");
   await slider.focus();
   await page.keyboard.press("ArrowRight");