fix(web): preserve user prompt line breaks

[swear01]

Closes #794

Summary

• Preserve single newlines in sent user prompt bubbles so Shift+Enter input displays as typed.
• Add an opt-in markdown plugin list with remark-breaks for user-authored prompt rendering only.
• Keep assistant/tool markdown rendering on the existing shared plugin list.

Root cause

User bubbles render through the markdown pipeline. Standard Markdown collapses single newlines unless remark-breaks or explicit hard breaks are used, while queued message previews use whitespace-pre-wrap.

Test plan

• git diff --check upstream/main..HEAD
• bun run --cwd web test src/components/AssistantChat/messages/user-bubble.test.tsx src/components/assistant-ui/markdown-text.test.ts
• bun run --cwd web typecheck
• bun run --cwd web test
• Manual HAPI test env from .git/info/hapi-local-notes.md: sent a multi-line user prompt and verified the user bubble preserves line breaks.

Pre-PR checks

• Checked open issues for similar reports; only bug(web): user prompt line breaks not rendered in chat bubble #794 matched.
• Checked open PRs for similar changes; none found.

[github-actions]

Findings

• No issues found with confidence >= 80% in the added or modified lines.

Summary

• Review mode: initial
• The change keeps hard-break parsing scoped to user prompt bubbles via preserveSingleLineBreaks, while assistant/tool markdown paths continue to use MARKDOWN_PLUGINS without remark-breaks.
• Residual risk: visual newline rendering is covered mostly through prop/plugin wiring, not a DOM-level assertion of rendered <br> output.

Testing

• Not run (automation: bun not found on PATH in this runner)

HAPI Bot