web: simplify buy credits panel. no custom purchase amount #5768

Job	Run time
build-and-check	2m 55s
test-integration-packages/billing	59s
test-integration-packages/internal	1m 12s
test-integration-common	39s
test-integration-cli	39s
test-integration-packages/agent-runtime	47s
test-cli	50s
test-packages/agent-runtime	42s
test-integration-web	38s
test-packages/billing	40s
test-packages/internal	47s
test-.agents	40s
test-web	42s
test-common	40s
test-integration-sdk	46s
test-sdk	48s
	14m 24s

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