feat: add code element support for whiteboard

[cosarah]

Summary

• Add wb_draw_code and wb_edit_code whiteboard actions for creating and editing syntax-highlighted code blocks
• Implement BaseCodeElement component with Shiki highlighting, IDE-style header, and line-by-line typing/insert/delete/replace animations
• Integrate into agent orchestration: tool schemas, director prompt context, prompt builder guidelines
• Fix whiteboard zoom intercepting code element scroll

Closes #383

Changes

Type system (2 files)

• lib/types/slides.ts: CODE enum, CodeLine interface, PPTCodeElement type
• lib/types/action.ts: WbDrawCodeAction, WbEditCodeAction, registered in SYNC_ACTIONS

Execution engine (1 file)

• lib/action/engine.ts: executeWbDrawCode (create code block with typing delay) and executeWbEditCode (insert_after, insert_before, delete_lines, replace_lines)

Rendering (3 files)

• BaseCodeElement.tsx: 630-line component — Shiki singleton, per-line HTML parsing, typing reveal with human-like easing, mount gating, replace/delete animations, scroll isolation from whiteboard zoom
• ScreenElement.tsx: register CodeElement in type map
• inline-action-tag.tsx: Code2 / FileCode icons for chat action tags

Agent orchestration (5 files)

• tool-schemas.ts: LLM action descriptions with parameter docs
• registry/types.ts + store.ts: add to WHITEBOARD_ACTIONS
• director-prompt.ts: WhiteboardActionRecord union, summarization cases
• prompt-builder.ts: action guidelines, Code Element Layout section, summarizeElement case, virtual whiteboard context

Other (1 file)

• element-fingerprint.ts: add code case for exhaustive switch

Code Review Results

Assessment: Ready to merge — no Critical issues found.

Low-priority observations (not blocking):

• replace_lines assumes contiguous line IDs (matches LLM prompt guidance)
• Module-level lineIdCounter is client-side only; Date.now() suffix prevents collisions
• Shiki language validation relies on outer try/catch fallback (functionally correct)
• No dark mode support (out of scope for this PR)

Test plan

• tsc --noEmit passes with zero errors
• pnpm dev starts without build errors
• Whiteboard code element renders with syntax highlighting and header
• Code element scroll works independently from whiteboard zoom
• Scroll falls through to whiteboard zoom at content boundaries
• Inline action tags show correct icons for wb_draw_code / wb_edit_code

🤖 Generated with Claude Code

[wyuc]

Tested locally, LGTM :)