chore: remove assembleFrontend from webserver, its already done in shadowjar

[AcevedoR]

it'll help incremental build by not building the UI for tests

[github-actions]

🐋 Docker image

ghcr.io/kestra-io/kestra-pr:14966

docker run --pull=always --rm -it -p 8080:8080 --user=root -v /var/run/docker.sock:/var/run/docker.sock -v /tmp:/tmp ghcr.io/kestra-io/kestra-pr:14966 server local

🧪 Java Unit Tests

	Tests	Passed ✅	Skipped ⚠️	Failed	Time ⏱
Java Tests Report	3807 ran	3793 ✅	14 ⚠️	0 ❌	1h 37m 50s 521ms

[github-actions]

Tests report quick summary:

success ✅ > tests: 3807, success: 3793, skipped: 14, failed: 0

unfold for details

Project	Status	Success	Skipped	Failed
cli	success ✅	80	0	0
core	success ✅	2005	0	0
executor	success ✅	4	0	0
jdbc	success ✅	11	0	0
jdbc-h2	success ✅	473	0	0
jdbc-mysql	success ✅	476	0	0
jdbc-postgres	success ✅	476	0	0
processor	success ✅	7	0	0
runner-memory	success ✅	1	0	0
scheduler	success ✅	23	0	0
script	success ✅	12	0	0
storage-local	success ✅	64	0	0
webserver	success ✅	171	0	0
worker	success ✅	4	0	0

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Develocity build scan: https://develocity.kestra.io/s/wfun5xndmccpo