fix(heatmap): prevent threadleak on shutdown

[wakonig]

Description

[Provide a brief description of the changes introduced by this pull request.]

Related Issues

[Cite any related issues or feature requests that are addressed or resolved by this pull request. Link the associated issue, for example, with fixes #123 or closes #123.]

Type of Change

• Change 1
• Change 2

How to test

• Run unit tests
• Open [widget] in designer and play around with the properties

Potential side effects

[Describe any potential side effects or risks of merging this PR.]

Screenshots / GIFs (if applicable)

[Include any relevant screenshots or GIFs to showcase the changes made.]

Additional Comments

[Add any additional comments or information that may be helpful for reviewers.]

Definition of Done

• Documentation is up-to-date.

[codecov]

Codecov Report

❌ Patch coverage is 62.79070% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bec_widgets/widgets/plots/heatmap/heatmap.py	62.79%	10 Missing and 6 partials ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

Benchmark comparison

Threshold: 20% (lower is better).
Result: 0 regression(s), 0 improvement(s) beyond threshold.

No benchmark regression exceeded the configured threshold.

No benchmark improvement exceeded the configured threshold.

All benchmark results

Benchmark	Baseline	Current	Change	Status
BEC IPython client with companion app	1.99235 s	1.96871 s	-1.19%	ok
BEC IPython client without companion app	2.00405 s	2.00237 s	-0.08%	ok
Import bec_widgets	0.01367 s	0.0130233 s	-4.73%	ok
tests/unit_tests/benchmarks/test_dock_area_benchmark.py::test_add_waveform_to_dock_area	0.153213 s	0.153985 s	+0.50%	ok