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/copilot-render.js

@@ -464,6 +464,7 @@ function attachmentIcon(att) {
   const n = (att.name || '').toLowerCase();
   if (m.startsWith('image/'))                          return 'bi-file-earmark-image';
   if (m === 'application/pdf' || n.endsWith('.pdf'))   return 'bi-file-earmark-pdf';
+  if (/\.(docx?|odt|rtf)$/.test(n))                    return 'bi-file-earmark-word';
   if (m.startsWith('audio/'))                          return 'bi-file-earmark-music';
   if (m.startsWith('video/'))                          return 'bi-file-earmark-play';
   if (m.startsWith('text/') || /\.(md|txt|csv|json|ya?ml|rs|js|ts|py)$/.test(n)) return 'bi-file-earmark-text';