getsentry/spotlight E2E: body.textContent() races React hydration — use web-first nav selector instead

Trap: body.waitFor() then body.textContent() !== '' looks like a safe readiness check because the body element exists immediately. Fix: the body element is present in the SPA HTML shell before React hydrates, so a one-shot read races React's first render. Other tests pass incidentally due to sendTestEnvelope 500ms settle delay or explicit waitForTimeout(1000). Stable readiness selector: nav[aria-label="Navigation"] in TelemetrySidebar.tsx — renders unconditionally once React mounts, independent of event data. Use a waitForAppReady(page, timeout) helper in fixtures.ts that asserts this nav locator is visible with Playwright's built-in auto-retry.