fix(a11y): fix contrast, labels and heading order, and scan both UIs in CI

[JustMaris]

axe-core (WCAG 2.2 A/AA plus best-practice, light and dark) found 30 violations across the website and web UI. Fixed:

• Website: links in body text were colour-only (now underlined); the broker directory region/category selects had no accessible name.
• Web UI: muted text, success/danger badges and accent links failed 4.5:1 in one or both themes. New --accent-solid token for fills behind white text, lighter dark-theme --accent for text; the hard-coded text-accent colour now uses the token. Broker filter selects got aria-labels; History and Action Needed headings no longer skip levels.
• New Accessibility CI job (part of Required checks passed) runs scripts/a11y/run.sh: builds the site and the web UI, seeds fictional data (scripts/a11y/seed), scans 15 pages in both themes. Entrance animations are disabled during the scan since axe reads mid-fade colours. Dependabot covers the npm deps.

Local result: no violations. This only covers what axe can detect automatically; keyboard and screen-reader behaviour still need a manual pass.