Fix arrow panel appearing behind marker tooltips

[fatadel]

Fixes#5913.

Tooltips are rendered via portal into #root-overlay, which is a sibling of #root. Two stacking contexts were trapping the arrow panel inside #root, preventing it from competing with tooltips:

• #root had z-index: 0, creating a stacking context (it's a flex item)
• .dragAndDropArea had isolation: isolate

This PR removes both so the arrow panel's z-index: 5 reaches the body stacking context. It also adds z-index: 4 to tooltips so they remain above regular content but below arrow panels and context menus (z-index: 5).

---

I know that removing z-index from the root may seem quite a significant change that can cause regressions. However, I don't see other solutions to this issue apart from a JS-based one by adding a portal to render the arrow panel inside #root-overlay but that seems overly complex and unnecessary to me.

My monkey testing didn't reveal any visual regressions, and Claude also claims there shouldn't be any.

Profile before / Profile after

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.41%. Comparing base (b8aabaa) to head (92dff0c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #5926 +/- ##
=======================================
Coverage 85.41% 85.41% =======================================
Files 321 321 Lines 32158 32158 Branches 8876 8782 -94 =======================================
Hits 27469 27469 Misses 4253 4253 Partials 436 436 

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[canova]

Nice, thanks!