ci: consolidate workflows into ci.yml #1

Job	Run time
Lint, type-check & markdown	1m 40s
Unit tests	1m 5s
Visual regression (Chromatic)	0s
E2E tests	0s
Build website (mock data)	-1s
Page visual snapshots (Playwright + Chromatic)	-1s
Lighthouse audit	-1s
	2m 42s

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