[7400] Overflow menu is clipped by Expert

[n-lark]

Description

Raises the tab-overflow dropdown's z-index (50 → 200) so it floats above the Expert drawer instead of being clipped. No repositioning logic, ff-tabs is generic and shouldn't know about the drawer, and the menu already clamps to the viewport.

Follow-up #7405 opened to centralize our scattered z-index values.

Related Issue(s)

Resolves #7400

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 76.70%. Comparing base (00ac964) to head (655e232).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7406   +/-   ##
=======================================
  Coverage   76.70%   76.70%           
=======================================
  Files         410      410           
  Lines       20887    20887           
  Branches     5067     5067           
=======================================
  Hits        16022    16022           
  Misses       4865     4865           

Flag	Coverage Δ
backend	76.70% <ø> (ø)

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