fix: heatmap small fixes

[dogfrogfog]

Brief

Changes

• fix heatmap test warning
• fix 404 error after /exemplars/single, /exemplars/merge pages reload
• add ability to start selection from already drawn selected area

heatmap.mov

Concerns

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
webapp/public/assets/app.js	443.6 KB (+0.01% 🔺)	8.9 s (+0.01% 🔺)	4.8 s (+18.51% 🔺)	13.7 s
webapp/public/assets/app.css	17.08 KB (0%)	342 ms (0%)	0 ms (+100% 🔺)	342 ms
webapp/public/assets/styles.css	9.49 KB (0%)	190 ms (0%)	0 ms (+100% 🔺)	190 ms
packages/pyroscope-flamegraph/dist/index.js	92.16 KB (0%)	1.9 s (0%)	1.2 s (+1.2% 🔺)	3.1 s
packages/pyroscope-flamegraph/dist/index.node.js	92.05 KB (0%)	1.9 s (0%)	700 ms (-18.09% 🔽)	2.6 s
packages/pyroscope-flamegraph/dist/index.css	7.37 KB (0%)	148 ms (0%)	0 ms (+100% 🔺)	148 ms

[codecov]

Codecov Report

Base: 66.40% // Head: 66.63% // Increases project coverage by +0.23% 🎉

Coverage data is based on head (c59503f) compared to base (7e82622).
Patch coverage: 77.78% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1545      +/-   ##
==========================================
+ Coverage   66.40%   66.63%   +0.23%     
==========================================
  Files         147      147              
  Lines        4997     4998       +1     
  Branches     1356     1155     -201     
==========================================
+ Hits         3318     3330      +12     
+ Misses       1674     1664      -10     
+ Partials        5        4       -1     

Impacted Files	Coverage Δ
...ipt/components/Heatmap/useHeatmapSelection.hook.ts	35.96% <0.00%> (+3.01%)	⬆️
webapp/javascript/components/Heatmap/index.tsx	83.75% <100.00%> (+10.92%)	⬆️
webapp/javascript/components/Heatmap/utils.ts	57.15% <100.00%> (ø)
webapp/javascript/services/exemplarsTestData.ts	100.00% <100.00%> (ø)
webapp/javascript/ui/Box.tsx	63.89% <0.00%> (ø)
webapp/javascript/ui/Button.tsx	81.82% <0.00%> (+0.57%)	⬆️

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[eh-am]

/create-server

[Rperry2174]

LGTM