docs: replicate the Island UI style in Nextra

[codeacme17]

Context

PR #96 migrates the visualization and container documentation to Nextra. The Nextra documentation should reproduce the established Island documentation UI exactly instead of introducing a new visual language.

The existing Island site under docs/ is the visual source of truth.

Requirements

• replicate the Island documentation shell: header, navigation, sidebar, table of contents, content widths, page spacing, footer, search, locale switcher, and theme controls
• replicate Island typography, colors, backgrounds, borders, radii, shadows, focus states, and responsive breakpoints
• replicate Island content styling for headings, paragraphs, links, lists, tables, code blocks, callouts, API references, and interactive component examples
• remove bespoke Nextra-only visual treatments that do not exist in the Island documentation, including the separate “signal lab” presentation
• preserve English and Chinese routes, light and dark themes, mobile behavior, static export, and component-demo functionality
• do not add another general-purpose UI framework

Acceptance criteria

• matched Island and Nextra routes have no unintended visible differences at equivalent desktop and mobile viewports
• visual comparison covers at least 1440×900 and 390×844 in light and dark themes
• header, sidebar, table of contents, content, API tables, code examples, and runnable component demos match the Island UI on every migrated page
• all English and Chinese component routes remain usable and responsive
• pnpm lint and pnpm verify pass

Related work

• docs: migrate visualization and container documentation #82
• docs: migrate visualization and container documentation #96