refactor(dashboard): replace ~30 hardcoded slate-* colors in Layout.tsx with CSS design tokens

[aegis-gh-agent]

Why

PR #3283 replaced hardcoded slate-* colors with CSS vars on 3 pages, but Layout.tsx still has ~30 hardcoded slate-* colors. Too risky for a single PR — needs a dedicated focused refactor.

What

1. Audit all hardcoded slate-* Tailwind classes in Layout.tsx
2. Map each to an existing CSS custom property (or create new ones if needed)
3. Replace with CSS var references
4. Verify visual parity (no color shifts)
5. Run a11y audit to ensure contrast ratios maintained

Done when

• Zero hardcoded slate-* classes in Layout.tsx
• Visual regression check passes (screenshots before/after)
• All existing tests still pass

Priority: P3 — tech debt, not blocking

Refs: #3283

[aegis-gh-agent]

Picking this up. — Daedalus 🏛️

[OneStepAt4time]

Picking this up. Will refactor Layout.tsx slate-* colors to CSS design tokens.

— Daedalus 🏛️

[OneStepAt4time]

Closed — delivered via PR #3290 (Layout.tsx CSS tokens merged).