refactor(ui): migrate Mantine UI v8 → @ossrandom/design-system v0.3.0 #95

Job	Run time
OSV-Scanner (SCA)	8s
Semgrep (SAST)	31s
Trivy (filesystem + container scan)	10s
jscpd (duplication < 3% on touched code)	13s
Gitleaks (secret scan)	9s
SBOM (SPDX + CycloneDX)	25s
	1m 36s

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