feat(chat): export assistant responses

[luoye520ww]

Summary

Add an export menu to completed assistant responses so answers can be shared without exposing raw Markdown.

Changes

• add an export action beside the existing response copy action
• export completed answers as PDF, DOCX, PNG, or HTML
• reuse the existing Write document renderer and save dialog
• allow the existing export IPC contract to render titled content without a source file
• add PNG support to the shared Write export pipeline
• add English and Chinese labels and focused coverage

Tests

• npm.cmd test -- src/main/ipc/app-ipc-schemas.test.ts src/main/services/write-export-service.test.ts src/renderer/src/components/chat/MessageTimeline.tool-summary.test.ts
• npm.cmd run typecheck
• npm.cmd run build
• focused ESLint
• git diff --check

Fixes #699

[XingYu-Zhong]

Reviewed locally. Focused export service, IPC schema, and chat timeline tests passed; no blocking issues found.