Unify docs code block line numbers

Goal

Make Plate docs code blocks match the shadcn-style code surface by showing line numbers for multi-line source snippets.

Findings

• rehype-pretty-code already supports showLineNumbers metadata and the CSS already styles [data-line-numbers].
• Many MDX fences opt into showLineNumbers, but generated/client code blocks still render without line numbers.
• apps/www/src/components/codeblock.tsx and InstallationCode force showLineNumbers={false}.
• Command-style snippets should stay clean; line numbers are useful for multi-line source snippets, not single-line install commands.
• Root cause for MDX blocks: code rendered with data-line-numbers, but lines were only class="line", while the docs CSS increments counters on [data-line].

Plan

1. Add showLineNumbers metadata to multi-line MDX source blocks before rehype-pretty-code runs.
2. Enable line numbers in client-rendered code blocks when the code spans multiple lines.
3. Verify typecheck, lint, and browser screenshots.

Progress

• 2026-05-25: Located MDX and client code block implementations.
• 2026-05-25: Added automatic showLineNumbers metadata for multi-line non-command MDX source blocks.
• 2026-05-25: Enabled line numbers for multi-line client-rendered code blocks and installation source snippets.
• 2026-05-25: Added data-line in rehype-pretty-code onVisitLine so the existing CSS counter renders.
• 2026-05-25: Verified with pnpm install, pnpm --filter www build:source, pnpm --filter www typecheck, and pnpm lint:fix.
• 2026-05-25: Browser verification on localhost:3001/docs/installation/react: React install code block shows line numbers 1-6; HTML contains both data-line-numbers and data-line.
• 2026-05-26: Regression found on localhost:3001/docs/plugin-shortcuts: lines had data-line, but generated code lacked data-line-numbers, so the CSS counter never started.
• 2026-05-26: Fixed the MDX pipeline to preserve a private __showLineNumbers__ flag across rehype-pretty-code and apply data-line-numbers to each generated code element after dual-theme expansion.
• 2026-05-26: Verified localhost:3001/docs/plugin-shortcuts in Browser: the MyDocumentPlugin block has data-line-numbers, 31 data-line rows, and visible line numbers.
• 2026-05-26: Verified with pnpm install, pnpm --filter www build:source, pnpm --filter www typecheck, pnpm lint:fix, and git diff --check.

Compound Evaluation

This produced reusable knowledge: showLineNumbers metadata is not sufficient unless line nodes match the local CSS selector. Updated docs/solutions/ui-bugs/2026-05-25-docs-code-block-light-theme-must-use-light-shiki-tokens.md.