feat: add gap handlers for flex and grid containers

[SmrutiSawant]

Add on-canvas gap handles

Flex and grid containers could only have gap changed from the Layout panel.
Padding and margin already have draggable on-canvas handles; this adds the
equivalent for gap.

Demo

Recording.2026-08-31.130737.mp4

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "fix: gap handles land on wrong seams whe..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.06%. Comparing base (12c787a) to head (a22330b).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #792   +/-   ##
========================================
  Coverage    66.06%   66.06%           
========================================
  Files           93       93           
  Lines        10386    10386           
========================================
  Hits          6862     6862           
  Misses        3524     3524           

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