docs: UI mockup / design docs for worker portal modules

Source: documentation-completeness.adoc plan, Status row "Not started".

The worker portal exposes 9 modules (case search, case detail, dashboards, action handlers, etc.) but there are no design docs or UI mockups capturing intended layouts, htmx interaction patterns, and accessibility patterns. Useful for jurisdiction onboarding and portal-modification reviews.

Acceptance: docs/modules/ROOT/pages/design/worker-portal-{module}.adoc for each module with wireframe (Excalidraw / Mermaid wireframe), interaction notes, and links to source templates. Post-UAT polish.