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.

CHANGELOG.md

@@ -10,6 +10,13 @@ release PR may merge — and a section is closed at the commit that bumps it.
 
 ### Added
 
+- The file viewer opens **word-processor documents** (`.docx`, `.doc`, `.odt`, `.rtf`):
+  when LibreOffice is installed on the server they are converted to PDF and shown as the
+  document, live-reloading when the file changes, exactly like a compiled `.tex`. A
+  document kept only inside the user's container is converted too — the server pulls a
+  copy out and converts that. With no LibreOffice the page says so and offers the
+  download, as before. The download button still saves the original document, not the
+  preview PDF.
 - Z.AI's new models are selectable on the **Models** page: **GLM-5.3** and **GLM-5.3-Flash**,
   both with a 1M-token context. GLM-5.3-Flash is natively multimodal, so images and videos
   attached to a message are sent to it directly instead of as a file path. Both always think —