feat: add code element support for whiteboard

[cosarah]

Summary

Add the ability to create and edit syntax-highlighted code blocks on the whiteboard, enabling live-coding demonstrations during lessons.

Features

• wb_draw_code action: Create code blocks on the whiteboard with syntax highlighting (Shiki, 17 languages supported)
• wb_edit_code action: Line-level editing operations (insert_after, insert_before, delete_lines, replace_lines) with smooth animations
• BaseCodeElement component: IDE-style rendering with typing animations, line numbers, macOS-style header bar
• Agent integration: LLM tool schemas, director context summarization, prompt guidelines for live-coding patterns

Scope

Category	File	Change
New	components/.../CodeElement/BaseCodeElement.tsx	New component with Shiki highlighting and typing animations
Types	lib/types/slides.ts	CODE enum, CodeLine, PPTCodeElement
Types	lib/types/action.ts	WbDrawCodeAction, WbEditCodeAction, add to SYNC_ACTIONS
Engine	lib/action/engine.ts	executeWbDrawCode, executeWbEditCode
Schemas	lib/orchestration/tool-schemas.ts	Action descriptions for LLM
Registry	lib/orchestration/registry/types.ts or store.ts	Add to WHITEBOARD_ACTIONS
Director	lib/orchestration/director-prompt.ts	Summarize code actions in agent context
Prompts	lib/orchestration/prompt-builder.ts	Code element usage guidelines
UI	components/chat/inline-action-tag.tsx	Code / Edit Code icon tags
Routing	components/slide-renderer/Editor/ScreenElement.tsx	Register CodeElement
i18n	lib/i18n/locales/*.json	code.untitled, code.lines keys

Notes

• Shiki is already installed (v3.21.0) — no new dependency needed
• i18n keys need to be added to all locale files (zh-CN, en-US, ja-JP, ru-RU)