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:

docs/shared-folders.md

@@ -38,7 +38,7 @@ Two things worth knowing about membership:
 There is no file explorer for shared folders — no grid of files, no upload button. The files live on the server, and members reach them through the assistant:
 
 - **Ask the assistant** — "what's in the recipes folder?", "add this note to documents", "send me the manual for the boiler". The assistant knows which folders you belong to, can list their contents, open and search files, and — if you have read & write — create and edit them.
-- **Open a file** — when the assistant shows you a file from a shared folder, it opens in the usual file viewer (Markdown rendered, images, PDFs, text), exactly like any other file. You can read it there; editing in the viewer is available if you have read & write access.
+- **Open a file** — when the assistant shows you a file from a shared folder, it opens in the usual file viewer (Markdown rendered, images, PDFs, syntax-colored code, text), exactly like any other file. You can read it there; editing in the viewer is available if you have read & write access.
 
 A practical consequence: if a member wants a file *from* a shared folder, the assistant is the way to get it — there is no download button on the folder itself. (An admin can of course reach the folder directly on the server, but members should not need to.)