Fix inbox header backdrop blur

[thomaspblock]

Summary

• add one shared inbox header backdrop across the list/detail grid
• render the list/detail header content transparent so the blur does not get clipped per column
• add Playwright coverage to assert there is exactly one full-width blurred inbox header backdrop

Tests

• just desktop-check
• cd desktop && pnpm exec playwright test --project=smoke home-collapsed-top-chrome.spec.ts

Draft for review.