feat(client): improve chat UI shortcuts and markdown rendering robustness

[rcyw]

Summary

This PR improves apps/client chat and right-panel UX, and hardens markdown rendering behavior for common LLM output patterns.

Changes

• Chat input

  • Replace send icon with upward arrow.
  • Add send shortcut tooltip (Enter keycap).
  • Align stop button visual style with the send button.

• Shortcut tooltip UX

  • Add reusable ShortcutTooltipContent.
  • Add platform-aware shortcut formatter (mod => ⌘ on macOS, Ctrl otherwise).
  • Add shortcut tooltips for panel collapse/expand, fullscreen close (Esc), and right-panel tab cycle (Cmd/Ctrl + Shift + V).

• Assistant response readability

  • Add markdown tone support and apply high-contrast tone for assistant merged text.
  • Slightly increase assistant step section contrast.

• Markdown normalization robustness

  • Extract markdown normalization into lib/markdown-normalize.ts.
  • Normalize 1) ordered list markers and immediate nested bullets.
  • Protect fenced code sections (including nested fences in ```md samples).
  • Auto-close unbalanced fences safely.

• Tests

  • Update SSE summary tests (message.part.updated non-loggable expectation).
  • Add markdown-normalize test coverage for list/fence edge cases.
  • Add shortcut unit tests for platform mapping and key-sequence formatting.

• Docs

  • Update CHANGELOG.md under [Unreleased].

Validation

• pnpm --filter @cofy-x/deck-app typecheck
• pnpm --filter @cofy-x/deck-app test

Both passed.