Shared UI patterns and consistency pass across features

[jeffpaul]

Description

Create a shared UI kit and perform a plugin-wide consistency pass to ensure all AI features look, feel, and behave consistently. This includes reusable UI components, unified loading and empty states, shared language patterns, and full accessibility coverage.

Goals

• Add /packages/ui/ with reusable components such as GenerateButton, SuggestionsList, ApplyWithUndo, LoadingSkeleton, ErrorNotice, and EmptyState.
• Define and document shared UI copy guidelines and component usage examples.
• Adopt a single design system for loading, success, and error states.
• Ensure all features (Excerpt Generation, Alt Text Generation, Summarization, Contextual Tagging) use the shared components rather than custom ones.
• Audit all UI strings for clarity, tone, and consistency.
• Verify full accessibility support – focus order, ARIA labels, keyboard navigation, and screen-reader announcements.
• Provide storybook-style previews or screenshots demonstrating component variations.

Acceptance Criteria

• Each feature module successfully adopts shared UI components.
• Loading, empty, and error states follow the same patterns and visual language.
• All UI strings are translatable and use a consistent glossary of terms.
• Accessibility testing passes for all UI interactions.
• Documentation exists in /docs/ui-guidelines.md covering design tokens, copy rules, and accessibility notes.

[jeffpaul]

Let's make sure whoever picks this up also includes #103 in their effort.