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/components/shared/file-explorer.js

@@ -329,7 +329,7 @@ export class FileExplorer extends LightElement {
       zip: 'bi-file-zip', gz: 'bi-file-zip', tar: 'bi-file-zip',
       mp3: 'bi-file-music', wav: 'bi-file-music', ogg: 'bi-file-music',
       mp4: 'bi-file-play', mov: 'bi-file-play', webm: 'bi-file-play',
-      doc: 'bi-file-word', docx: 'bi-file-word',
+      doc: 'bi-file-word', docx: 'bi-file-word', odt: 'bi-file-word', rtf: 'bi-file-word',
       xls: 'bi-file-excel', xlsx: 'bi-file-excel', csv: 'bi-file-excel',
     };
     return map[ext] ?? 'bi-file-earmark';