feat(file-viewer): syntax highlighting for code files and chat code blocks

Vendor highlight.js (core + python, javascript, typescript, json, yaml,
bash) and highlight the file viewer's text kind (computed once per load)
plus fenced blocks in renderMarkdown. Colors come from new --syn-* CSS
variables aliased to the existing palette, so dark mode follows.

docs/projects.md

@@ -40,6 +40,8 @@ A file explorer rooted at the project folder:
 - Click a **folder** to navigate into it.
 - The listing **updates by itself**: if another member or the assistant creates, renames or deletes a file while you're looking at a folder, the change appears within a second — no refresh needed.
 
+For **code files** — Python, JavaScript, TypeScript, JSON, YAML, shell scripts — the viewer colors the syntax (keywords, strings, comments…), so the structure of a script is easy to follow when reviewing it. The same coloring applies to code blocks inside chat messages.
+
 For **PDF** files, the viewer shows the whole document as one continuous scroll — every page, in order, on phone, tablet and computer alike. A small toolbar on top gives zoom out / zoom in and tells you which page you are on (`3 / 12`). The text stays selectable and copiable where the PDF itself has real text. Pages are drawn as you reach them, so a long document opens quickly instead of making you wait for the last page. If you'd rather open it in another app, the **download** button in the header saves the original file.
 
 For **Markdown** files (`.md`), if you have write access the viewer has two tabs: