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: 4/5

The PR is not safe to merge until gap bands handle row-spanning grid items without losing row boundaries.

A row-spanning item can merge distinct visual rows during clustering, preventing the corresponding row-gap handle from rendering.

Files Needing Attention: frontend/src/components/GapHandler.vue

Prompt To Fix All With AI

### Issue 1
frontend/src/components/GapHandler.vue:172-179
**Row spans merge gap bands**

When a grid item spans multiple rows, `clusterLines` groups ordinary items from both rows through the spanning box, causing the row-gap handle between those rows to disappear.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (3): Last reviewed commit: "Merge branch 'develop' into gaps" | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.06%. Comparing base (a8264b7) to head (e6b9906).
⚠️ Report is 2 commits behind head on develop.

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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[surajshetty3416]

@mergify update

[mergify]

update

✅ Branch has been successfully updated

[greptile-apps]

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