feat(app): add copy rich text option for message content

[anduimagui]

Issue for this PR

Closes #10693

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes assistant response copy behavior for desktop so rich text paste targets (for example Apple Mail) no longer fall back to Times when copying from rendered markdown.

It adds explicit clipboard payloads (text/plain + text/html) for markdown selection copy and assistant rich copy, includes inline fallback font/link/code styles in copied HTML, and introduces a desktop settings control for assistant copy mode (Plain text, Rich text, Ask each time) with rich text as the default for new settings. The settings row was also adjusted so the selector stays aligned on the right, and the copy button icon remains consistent across modes.

How did you verify your code works?

• Ran bun test src/components/markdown-copy.test.ts in packages/ui
• Ran bun test src/components/session-review-search.test.ts in packages/ui
• Ran bun test src/i18n/parity.test.ts in packages/app
• Push hook bun turbo typecheck passed before push

Screenshots / recordings

Copy options in settings:

Message copy button dropdown in chat:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found one potential related PR:

PR #11638: feat(tui): add rich text clipboard support for copying formatted content
#11638

This PR may be related as it also deals with rich text clipboard support, though it appears to be for TUI (terminal UI) while PR #16056 is focused on desktop. The PRs target different platforms but share the same core feature of handling rich text in clipboard operations with explicit payload formats.

[anduimagui]

yh this PR intentionally scopes to desktop/webview chat UI @adamdotdevin
no TUI codepaths touched

[andrewdunndev]

I just closed #19035 which overlapped with this PR. Your approach is cleaner -- dedicated utility modules, tests, and the settings toggle is a better UX than the per-message dropdown I was building.

Two things from my implementation that might be worth considering here:

1. Email-specific copy mode

Your rich text copy preserves the rendered styles, which works well for Notion/GitHub/Docs. But when pasting into Gmail or Outlook, the app-specific CSS classes don't render. I built an "email sanitization" pass that strips all classes/styles and inlines email-safe CSS directly on elements:

// Strip all classes/styles, then inline email-safe CSS
div.querySelectorAll("a").forEach(a => 
  a.setAttribute("style", "color: #1a73e8; text-decoration: underline;"))
div.querySelectorAll("code").forEach(code => {
  if (code.parentElement?.tagName !== "PRE")
    code.setAttribute("style", "background: #f1f3f4; padding: 1px 4px; border-radius: 3px; font-family: monospace;")
})
// ... pre, blockquote, table, th/td similarly
// Wrap in: <div style="font-family: Arial, Helvetica, sans-serif; font-size: 14px;">

This could be a third copy mode ("Copy for Email") or a variant of your rich text mode that detects the target. Either way, the inline styles are what make it render correctly in email clients.

2. DOMPurify before DOM parsing

Your cleanMarkdownHTML uses DOMParser on the rendered HTML. If the markdown contains untrusted content (model output with embedded HTML), setting innerHTML on even an off-DOM element can trigger network loads from <img>, <video>, etc. Running DOMPurify first (with FORBID_TAGS: ["img", "video", "audio", "iframe"] for the clipboard path) prevents this. The app already uses DOMPurify in Markdown.tsx so the dependency is there.

Happy to contribute either of these as a follow-up PR if this one lands, or as additions here if you'd prefer.