Add color to children

[MaHaWo]

• add background color to children card display in ChildrenGallery
• child database table has a color column now
• add color input element to the ChildrenRegistration component
• use color with and without image, since image gets removed on screens with small width
• adjust styles and tests as necessary
• same flaky test syndrome as the other PR, fixed in Add trafficlight feedback frontend #214
  fixes Add colour to children #154

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 49 lines in your changes missing coverage. Please review.

Project coverage is 1.51%. Comparing base (4cbc0c8) to head (44bb6fe).

Files with missing lines	Patch %	Lines
frontend/src/lib/components/ChildrenGallery.svelte	0.00%	37 Missing ⚠️
...end/src/lib/components/ChildrenRegistration.svelte	0.00%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #218      +/-   ##
========================================
- Coverage   1.52%   1.51%   -0.02%     
========================================
  Files         83      83              
  Lines       2488    2506      +18     
  Branches      95      95              
========================================
  Hits          38      38              
- Misses      2370    2388      +18     
  Partials      80      80              

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