fix: declare missing frontend dependencies in package.json

[Haiagari]

Problem Description

When building from source or running npm run build / pnpm build, TypeScript and Vite fail with 60+ missing module errors because several packages imported across src/ are not declared in package.json.

Specifically, imports like:

• @codemirror/language in src/surfaces/editorLanguage.ts
• @codemirror/state & @codemirror/view across editor surfaces
• @codemirror/autocomplete & @codemirror/commands
• @lezer/common
• mermaid in src/surfaces/mermaidPlugin.ts
• diff in src/lib/unifiedDiff.ts
• unified in src/surfaces/AgentMarkdown.tsx

Solution

Declare all missing dependencies in package.json. Verified that pnpm build (tsc && vite build) and tauri dev build cleanly with 0 errors.