test: add dashboard design-system regression coverage

[icebear0828]

Context

The dashboard has both light/dark themes and many repeated operational states. As the UI refactor moves styling into shared primitives, tests should prevent regressions in token output, contrast-sensitive selectors, dark-mode rendering, and semantic data display.

Depends on #497 and #498. Should be expanded as #499, #500, and #501 land.

Scope

• Add token/theme tests that verify key light and dark color values differ where intended and remain present in generated CSS.
• Add focused component tests for shared primitive variants: primary/secondary/destructive buttons, status badges, progress bars, cards/panels, tabs/chips, inputs/selects.
• Add page-level regression tests for high-risk semantics: usage history/window values, account status badges/toggles, error unread state, and proxy health states.
• Add a lightweight contrast check for known foreground/background pairs where practical.

Acceptance criteria

• Shared primitives have tests covering core variants and disabled/hover/selected state class output or rendered behavior.
• CSS/theme tests verify important light/dark token output and key selectors.
• Usage stats tests continue to protect history-window semantics.
• Tests are deterministic and do not require real external services.
• No TypeScript any is introduced.

Verification

• Run npm test.
• Run npm run build.
• If frontend styling changes are included, manually verify the dashboard in a browser after build.

Notes

Mock coverage is enough for this visual regression issue because it does not change external SDK/API behavior. Real-service E2E remains required for future changes that touch external services.