Accessibility follow-up: WCAG AA verification and remediation for dark mode

### Goal
Provide verifiable WCAG AA contrast/accessibility evidence for dark mode and restore compliance wording only after checks pass.

### Scope
- Configure and run **dark-mode-specific** pa11y/axe scans across composition component pages.
- Triage and fix reported issues (starting with color contrast and missing labels).
- Create a **contrast matrix** document for dark theme token pairs and key UI surfaces.
- Link evidence in PR/docs (pa11y report + contrast matrix).

### Deliverables
- [ ] Dark-mode pa11y report committed or attached as CI artifact
- [ ] Contrast matrix document added
- [ ] Critical/serious accessibility findings addressed or explicitly tracked
- [ ] Compliance statement updated only after evidence passes

### Acceptance criteria
- Automated dark-mode accessibility run completes successfully with agreed threshold.
- Contrast matrix exists and covers primary text/surface/state combinations used in dark theme.
- PR references both evidence artifacts (pa11y report + contrast matrix).