- Create server/src/services/placeholder-service.ts with addEntry, replaceEntry, listEntries
- Generates PLACEHOLDERS.md with Active Placeholders and Replaced markdown tables
- Add ChatPlaceholderEntry interface to packages/shared/src/types/chat.ts
- Export ChatPlaceholderEntry from packages/shared/src/index.ts
- Add markAsPlaceholder method to chatFileService in chat-files.ts
- Add ChatCodeFilePreview component with hljs syntax highlighting
- Fetch file content from contentPath with credentials
- Use DOMParser-based safe rendering (no dangerouslySetInnerHTML)
- Include copy button, language label, and ChatFileCard download below
- Add extToLang extension-to-language mapping
- Register 14 common languages with hljs
- Add highlight.js as direct dependency in ui/package.json
- Implement chatFileService(db) with create, getById, listByConversation, listByMessage, createReference, listReferences, attachToMessage
- Export deriveCategory() helper mapping MIME types to image/code/document/other
- Add unit tests verifying service methods and category derivation with mocked DB
