feat(files): in-app .docx preview + allow office/text mimes

- .docx now renders client-side via docx-preview (fetches bytes from our
  own storage; works with private MinIO/disk). Drops Microsoft's hosted
  Office viewer which can't reach a private object store.
- add office (.docx/.doc/.xlsx/.xls) + text/csv to PREVIEWABLE_MIMES so
  /api/v1/files/[id]/preview returns a URL instead of rejecting them
  (was surfacing as a misleading "Failed to load preview")
- legacy .doc + spreadsheets fall through to a download CTA (can't render
  client-side); text/csv use the existing TextPreview

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -80,6 +80,7 @@
     "country-flag-icons": "^1.6.17",
     "cron-parser": "^5.5.0",
     "date-fns": "^4.1.0",
+    "docx-preview": "^0.3.7",
     "drizzle-orm": "^0.45.2",
     "echarts": "^6.0.0",
     "echarts-for-react": "^3.0.6",