[6947] User menu shows behind pinned expert drawer

[n-lark]

Description

The header and overlay drawer both have z-index: 110. Since the drawer appears later in the DOM, it paints on top of the header's entire stacking context including any dropdowns inside it. Raising the header to z-index: 120 makes its stacking context win regardless of DOM order.

Related Issue(s)

Resolves #6947

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.29%. Comparing base (b14a552) to head (dea0b85).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6948   +/-   ##
=======================================
  Coverage   76.29%   76.29%           
=======================================
  Files         403      403           
  Lines       20293    20293           
  Branches     4880     4880           
=======================================
  Hits        15482    15482           
  Misses       4811     4811           

Flag	Coverage Δ
backend	76.29% <ø> (ø)

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