fix: column header overlay

[Schwehn42]

Description

Right now, the color picker isn't correctly rendered in Chrome.
Reason for this is a wrong z-index for column header, which would lead to these problems.
The z-index was introduced in #4391, fixing another issue concerning the emoji overlay.
However, the value itself was less than the notes z-index.

The proposed changes here adjust the z-index of column headers to always be greater than the notes z-index, so all components are explicitly overlayed.

Resolves #4538.

Changelog

• Added new z-index var $column-header-z-index which is greater than $note-z-index
• Applied $column-header-z-index to column header
• Change z-indices in components related to the color picker; if not required removed entirely

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4556.development.scrumlr.fra.ics.inovex.io
This deployment is only for testing purposes and will be deleted after 1 week.
To redeploy rerun the workflow.
DO NOT STORE IMPORTANT DATA ON THIS DEPLOYMENT

Deployed Images

• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-6b0e2e8

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-6b0e2e8

[octomind-dev]

🐙 Octomind

Test Report: 14/14 successful.

description	status	details
About Section Visibility Test	Passed ✅	click
change avatar	Passed ✅	click
check Privacy Policy	Passed ✅	click
check terms & conditions	Passed ✅	click
close cookie banner - front page	Passed ✅	click
close cookie banner - sign-in	Passed ✅	click
create and delete board columns	Passed ✅	click
create_and_delete_notes_and_actions_v2	Passed ✅	click
create lean coffee board	Passed ✅	click
edit_notes_and_actions_v5	Passed ✅	click
get started	Passed ✅	click
share_session	Passed ✅	click
sign-in	Passed ✅	click
test all ways to open the setup flow	Passed ✅	click

commit sha: 6b0e2e8

[mateo-ivc]

lgtm 👍🏽