refactor(ui): migrate Mantine UI v8 → @ossrandom/design-system v0.3.0 #101

Job	Run time
SBOM (SPDX + CycloneDX)	22s
Trivy (filesystem + container scan)	9s
OSV-Scanner (SCA)	7s
jscpd (duplication < 3% on touched code)	19s
Semgrep (SAST)	27s
Gitleaks (secret scan)	8s
	1m 32s

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