fix(dashboard): NotFoundPage uses raw Tailwind colors (bg-cyan, text-void)

[OneStepAt4time]

Bug

NotFoundPage uses raw Tailwind classes that don't participate in the design token system:

• bg-cyan → should use var(--color-accent-cyan)
• text-void → should use var(--color-void)
• hover:bg-cyan/80 → should use CSS var with opacity

File: dashboard/src/pages/NotFoundPage.tsx line 18

Impact: 404 page looks broken in light theme — cyan button text becomes invisible on light backgrounds.

Acceptance criteria:

• Replace all raw Tailwind color classes with CSS var equivalents
• Test in both dark and light themes

Labels: dashboard, bug

[OneStepAt4time]

Closed by PR #4069 (batch CSS variable migration, merged).