fix(chat): preserve pasted user message newlines

[AlphaCatMeow]

Linked issue

Closes #352

Depends-On: none
Stack-Root: #353

Summary

• Normalize CRLF/CR to LF without trimming or moving logical newlines.
• Insert pasted plaintext as one safely escaped, undoable contenteditable operation and serialize DIV/P/placeholder BR nodes without double-counting blank lines.
• Preserve leading/trailing newlines through draft, outbound payload, uploads metadata, history/replay, and user-message display.
• Add a user-only trailing-LF visual anchor so Chromium allocates the final line box without changing DOM text or assistant/tool Markdown semantics.
• Keep desktop GUI and Gateway WebUI behavior mirrored.

Change scope

• Modules: agent-gui React UI, Gateway WebUI, mirror verification, frontend tests
• Key paths:
  • crates/agent-gui/src/components/chat/MentionComposer.tsx
  • crates/agent-gui/src/lib/chat/composerText.ts
  • crates/agent-gui/src/lib/chat/messages/userMessageContent.tsx
  • crates/agent-gateway/web/src/components/chat/MentionComposer.tsx
  • crates/agent-gateway/web/src/lib/chat/composerText.ts
  • crates/agent-gateway/web/src/lib/chat/userMessageContent.tsx
  • mirrored Node and real-browser pipeline fixtures/tests

No Rust, Go, protocol, database, dependency, lockfile, or generated-code changes.

Rebase and diff audit

• Rebased without conflicts onto latest upstream/main@00a2c6fc on 2026-08-06.
• Final topology is exactly three PR commits on that baseline (upstream/main...HEAD = 0 behind / 3 ahead).
• Original and rebased implementation/docs patches are range-diff equivalent; the style commit only adds the post-rebase worklog/acceptance record beyond its original formatting patch.
• Original and rebased product scope remains 23 files. GatewayApp.tsx, useSendChatTurn.ts, and scripts/mirror-manifest.json were explicitly audited to preserve current-main behavior while adding only newline fidelity support.
• The accepted product tree at bb060885 and final PR head differ only in docs/worklog/paste-newline-serialization.md.

Screenshots / preview

Production-module runtime preview for alpha\n\nbeta:

Stage	GUI	Gateway WebUI
Clipboard logical newlines	2	2
Composer logical newlines	2	2
Outbound logical newlines	2	2
History/replay logical newlines	2	2
User bubble DOM logical newlines	2	2
User bubble visual lines	3	3
white-space	pre-wrap	pre-wrap

The attached PNG below uses the actual production composer and UserMessageContent modules and shows the five-stage escaped text/counts, sent bubble, and history/reconnect replay without user session data.

Verification

• GUI production build/typecheck: passed
• Gateway WebUI production build/typecheck: passed
• GUI focused paste-pipeline + user-bubble tests: 19/19 passed
• Gateway WebUI focused paste-pipeline tests: 5/5 passed
• Gateway WebUI full tests: 498/498 passed
• GUI full tests: 1404/1409 passed; all 5 failures reproduced exactly on a fresh non-Worktree snapshot of upstream/main@00a2c6fc:
  • 2 mention composer selection source-extractor CRLF assumptions
  • 2 mention refetch source-extractor CRLF assumptions
  • 1 existing Rust/TypeScript provider usage preset byte comparison
• Full lint baseline comparison on the same Windows CRLF checkout:
  • GUI current: 421 errors / 358 warnings / 9 infos; latest-main baseline: 424 / 358 / 9
  • WebUI current: 290 errors / 310 warnings / 10 infos; latest-main baseline: 292 / 310 / 10
  • changed production files pass targeted lint with no errors; normalized LF Git blobs pass Biome checks with exit 0
• Actual Chromium production-module matrix: passed for LF/CRLF/CR; zero/single/multiple blank lines; leading/trailing newlines; pure whitespace; rich + plaintext clipboard; Markdown; Unicode/emoji; 40,002-character input; undo/redo; repeat send; cancellation; reload/history replay; and GUI ↔ WebUI replay
• Same-HEAD Tauri desktop runtime: Rust dev build 747/747, current-workspace binary launched and responsive; user explicitly accepted the complete manual matrix on 2026-08-06
• node scripts/check-mirror.mjs: 116 files passed
• git diff --check upstream/main..HEAD: passed
• No requested validation was skipped.

Open PR overlap

• feat(chat): 双端对齐 Composer 工具栏与上下文计量 #158 and feat(chat): 输入栏增加 Web Speech API 语音输入 #276 remain open and overlap the mirrored MentionComposer.tsx files.
• No commits were cherry-picked and this PR has no dependency on either change.

Pre-submit checklist

• A requirement issue is linked.
• Rebased onto the latest target baseline with no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, personal data, generated assets, or local runtime files are included.
• Worklog and behavior tests are updated.
• Same-HEAD manual acceptance is complete.

[github-actions]

PR governance checks passed. Awaiting human review.