feat(viewer): preview word documents (.docx/.doc/.odt/.rtf) as PDF

The file viewer converts word-processor documents to PDF server-side via
LibreOffice (skald_core::docx::DocxConverter), mirroring the LaTeX pipeline
but content-hash cached: the format is self-contained, so there is no
dependency graph and the file watcher needs no expansion. Container-only
documents are shuttled out and converted on the host. With no LibreOffice
installed the viewer says so and falls back to download-only. Downloads
still save the original document, not the preview PDF.

src/frontend/api/ws.rs

@@ -40,9 +40,10 @@ The URL returned by image_generate already points to the correct endpoint — us
 Do NOT append \".png\" or any extension to the URL.\n\
 \n\
 FILES: To let the user look at a file directly, call show_file_to_user(path). Supported: \
-Markdown, source code, images (PNG/JPG/GIF/WebP/SVG), PDF, and LaTeX (.tex — auto-compiled \
-to PDF server-side). HTML opens in a new browser tab. Prefer this over pasting long file \
-contents into chat.";
+Markdown, source code, images (PNG/JPG/GIF/WebP/SVG), PDF, LaTeX (.tex — auto-compiled \
+to PDF server-side), and word-processor documents (.docx/.doc/.odt/.rtf — converted to \
+PDF server-side when LibreOffice is installed). HTML opens in a new browser tab. Prefer \
+this over pasting long file contents into chat.";
 
 const HELP_TEXT: &str = "\
 **Available commands**\n\n\