feat(markdown): Editor component optimization — maxLength, toolbar config, streaming, code block actions

[Copilot]

Five enhancements to the Markdown editor/renderer components: character limits, toolbar flexibility, SSE streaming support, toolbar icon fixes, and code block action buttons.

Changes

• maxLength prop — Truncates input at limit, preserves cursor position, renders 128 / 500 counter below editor when set
• Toolbar code/codeblock fix — code action now inserts `text` (was incorrectly inserting fenced blocks). codeblock gets a distinct SVG icon (rectangle with lines) vs text </> for inline code
• toolbar prop union type — Accepts false (hide), ToolbarItem[] (whitelist), or legacy { show, items } object. Normalized internally via normalizeToolbarConfig()
• SSE streaming — streaming prop bypasses debounce, appends blinking cursor via ::after pseudo-element, auto-scrolls when near bottom. onStreamEnd fires on streaming: true → false transition
• Code block download & preview — Buttons grouped in .code-block-actions container. Download maps data-language → file extension via LANG_EXT_MAP. Preview (HTML only) renders in sandbox="allow-scripts" iframe modal (no allow-same-origin)
• i18n — Added download, preview, closePreview strings for both locales
• Docs — README updated with new prop tables, toolbar config examples, SSE integration demo. Website adds streaming code example tab

Usage

// Streaming
<MarkdownRenderer
  source={content}
  streaming={isStreaming}
  onStreamEnd={() => console.log('done')}
/>

// Toolbar whitelist + character limit
<MarkdownEditor
  toolbar={['bold', 'italic', '|', 'code', 'codeblock']}
  maxLength={5000}
/>

// Hide toolbar
<MarkdownEditor toolbar={false} />

Types

export type ToolbarConfig = boolean | ToolbarItem[] | { show?: boolean; items?: ToolbarItem[] };

All new props are optional with backward-compatible defaults. Existing tests unaffected (3 pre-existing failures unchanged).

---

📱 Kick off Copilot coding agent tasks wherever you are with GitHub Mobile, available on iOS and Android.