[Bug] Preserve logical newlines across paste, serialization, and user bubbles

[AlphaCatMeow]

Affected area

Desktop UI (agent-gui / React) and Gateway WebUI.

Version or commit

main@7de95a20bf93cfe026a57f6367c453e74a50acef

Environment

• Windows 11 desktop app (pnpm --dir crates/agent-gui tauri dev)
• Self-hosted Gateway backend and Gateway WebUI
• The affected desktop React path is shared by Windows, macOS, and Linux

Steps to reproduce

1. Paste alpha\n\nbeta into the message composer. The clipboard may contain only text/plain or both text/plain and text/html.
2. Send the message and inspect the optimistic user bubble.
3. Reload the window or restore the same conversation from history.
4. Repeat with LF, CRLF, CR, leading/trailing newlines, and multiple consecutive blank lines.
5. Compare a manually typed message with the same logical text.

Expected behavior

• CRLF/CR may normalize to LF, but the number and position of logical newlines remain unchanged.
• A single blank line renders as one blank line.
• Composer draft, outbound payload, history/replay, desktop bubble, and Gateway WebUI bubble contain the same logical text.
• Manual input and paste produce identical payloads and rendering.

Actual behavior

• Chromium represents a pasted empty line as DIV > BR.
• The composer serializer counts both the block boundary and the placeholder BR, so alpha\n\nbeta becomes alpha\n\n\nbeta.
• Full-message trim() calls remove valid leading and trailing logical newlines.
• The user bubble faithfully renders the already-expanded text, making one blank line appear roughly three lines high.
• A trailing LF remains in DOM text but Chromium does not allocate its final visual line box.

Evidence

The deterministic production-module browser reproduction records every stage:

Stage	Before fix for alpha\n\nbeta	Expected
Clipboard	2 newlines	2
Composer	3 newlines	2
Outbound	3 newlines	2
History/replay	3 newlines	2
User bubble DOM	3 newlines	2

GUI and Gateway WebUI screenshots with the corrected five-stage counts and visual spacing are attached below.

Pre-submit checklist

• I searched existing issues and pull requests and found no duplicate issue.
• This is not a security vulnerability.