fix: correct remaining header layout regressions

- Fix #header-status-host mobile gap: flex: 0 1 auto -> flex: 0 0 0% so
  the empty host takes zero layout space regardless of content, preventing
  justify-content:space-between from creating invisible gaps on mobile

- Fix .banner ul.nav: add overflow:hidden + align-items:center so nav
  items cannot paint outside the 50px header boundary

- Fix sidebar tooltip position: relative -> position: absolute on
  .sidebar-tooltip-text, with position:relative on .menu-btn as anchor.
  The relative positioning put the hidden span in document flow, making
  the hamburger button ~56px tall and overflowing the 50px header.
  Tooltip now appears correctly to the right of the button on hover.

- Remove .playwright-mcp/ debug artifacts and header-*.png screenshots
  committed in the previous pass; add .playwright-mcp/ to .gitignore

Author: BenjaminMichaelis

.gitignore

@@ -10,6 +10,7 @@ TestResults/
 ~$*.do[ct]x
 ~*.tmp
 ~$*.dotm
+.playwright-mcp/
 
 # Files to keep (primarily book content)
 !.gitignore