UI Library docs: page-per-component + source-generated tables (3/3)

[librowski]

Part 3 of 3. Stacked on #43 (base branch librowski/ui-consumers) - review/merge #41 then #43 first. This PR's diff is only the documentation work.

Summary

Builds the UI Library documentation in apps/docs (Starlight): a page-per-component reference (twin to the old Overflow UI docs) with live interactive examples, props and CSS-variable tables generated from source (no hand-maintained tables), and a Design Tokens pipeline page.

Changes

Docs site

• React island support added (@astrojs/react); @workflowbuilder/ui styles loaded via Starlight customCss (safe - no global reset; tokens follow the Starlight light/dark theme).
• New UI Library sidebar section: Overview, Design tokens, UI Components (18 pages), Diagram Components (5 pages). Each component page has a live example (React island), usage, props, and CSS variables.
• Rebranded the remaining "Overflow UI" prose (architecture / design-system / faq) to @workflowbuilder/ui on Base UI.

Source-generated API tables

• apps/docs/scripts/generate-ui-api.mjs runs TypeDoc over the @workflowbuilder/ui barrel to extract per-component props (type / required / default / description), and extracts --ax-public-* CSS variables from the stylesheets, into a git-ignored ui-api.json. Rendered inline via PropsTable / CssVariablesTable Astro components. Wired into docs dev / build.
• To make TypeDoc extract cleanly, component prop types are now named + exported through the @workflowbuilder/ui barrel, with @default JSDoc tags. This is the only packages/ui change in this PR and is additive.

Design tokens

• New page documenting the tokens.json (Figma) → Style Dictionary → --ax-* CSS pipeline, theming, customization, regeneration, and the known "missing token" gaps.

Verification

• build:docs green (177 pages); docs typecheck has no errors in the new files (only the 4 pre-existing head.astro / sidebar.astro errors).
• Rendered in a browser: live examples interactive (switch toggles, modal opens), props/CSS tables render from generated data, theme follows Starlight.

Notes

• Props/CSS tables are fully source-driven - regenerated on every docs build, never drift.
• The "Overflow" product upsell card (overflow-card.astro) is intentionally left as-is (advertises the commercial product, not the vendored library).

[librowski]

Superseded by the rebranched series (no librowski/ prefix): #44 (packages), #45 (consumers), #46 (docs). Closed automatically when the head branch was renamed.