Why: Spacing-only refinements still looked like the original flat gray workspace and did not satisfy the approved visual redesign.
Evidence: 272 unit tests, 16 browser scenarios across 320-1920px, and the production build pass locally.
Impact: Gives navigation, authored messages, assistant results, and work details clearly different visual surfaces while preserving behavior.
Refs: Agent-System-Dev feature/workbench-visual-redesign
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