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.

web/i18n/en.js

@@ -1049,6 +1049,7 @@ export default {
   'fv.mode_source':         'Show source',
   'fv.binary_unavailable':  'Preview not available for this file type.',
   'fv.latex_failed':        'LaTeX compilation failed — showing source instead',
+  'fv.docx_failed':         'Document conversion failed',
   'fv.tab_view':            'View',
   'fv.tab_edit':            'Edit',
   'fv.dirty_badge':         'Unsaved changes',