feat: enable mermaid diagram export as SVG/PNG/MMD

[xmengnet]

Summary

Enable exporting mermaid diagrams from chat messages as SVG / PNG / MMD.

streamdown already ships a built-in download dropdown for diagram blocks
(gated by controls.mermaid.download); this change turns it on, overrides
its labels through the translations prop using the app's own i18n
(chat.markdown.diagram, en-US + zh-CN), and scopes the mermaid action-bar
CSS to direct child buttons so the dropdown items keep a visible hover
state.

Change type

• Feature

Affected areas

• Frontend / UI

Verification

• pnpm typecheck
• pnpm lint
• pnpm build
• Manual browser check: download menu shows on mermaid blocks; SVG, PNG
  and MMD downloads produce valid files (SVG text-editable, PNG 5x
  rasterized, MMD source intact); streaming disabled state, fullscreen
  and screenshot filtering unaffected.

Screenshots, API examples, or logs

Mermaid block action bar now shows a download button with SVG / PNG / MMD
options (labels localized: 下载图表 / 导出为 SVG / 导出为 PNG / 导出为 MMD).

Configuration, migration, and compatibility notes

No config, env, schema, API contract, or deployment changes. Download
filenames are fixed by the upstream library (diagram.svg / diagram.png /
diagram.mmd).

Documentation

• Documentation is not needed for this change.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.