feat(ui): add responsive admin navigation and reusable admin component library #693

Parent epic: #679

Evidence

Several admin pages are 800–1122 lines and repeat tables, filters, banners, dialogs, and form patterns. The shell uses a fixed ~220px sidebar without a complete responsive drawer pattern; tenant switching is visually present but disabled.

Objective

Create an accessible responsive admin foundation and migrate a small representative slice without a full redesign.

Scope

• Add responsive sidebar/drawer, focus management, skip link, keyboard close, and active-route semantics.
• Add reusable DataTable, Pagination, FilterBar, ActionBanner, ConfirmDialog, FormField, and StatusBadge primitives.
• Standardize loading, empty, error, forbidden, and success states.
• On single-tenant deployments show a non-interactive tenant badge; show a switcher only when capability/authorization exists.
• Migrate two representative large pages and document adoption.
• Preserve progressive enhancement and server-rendered/offline behavior.

Acceptance criteria

• Layout works at 320px, tablet, desktop, zoom 200%, and keyboard-only use.
• Components meet WCAG 2.2 AA semantics/contrast/focus requirements.
• Destructive actions require clear confirmation and safe feedback.
• E2E and automated accessibility smoke tests cover navigation and migrated pages.
• No authorization decision relies on hidden/disabled UI alone.