Add OnlyOffice-based preview for office documents

Replace Aspose.Words with OnlyOffice Document Server for generating
bitmap previews of office documents (Word, Excel, PowerPoint, etc.).

Backend:
- Add cista/onlyoffice.py conversion client
- Convert office docs directly to PNG via OnlyOffice, then AVIF via pyvips
- Make office previews optional based on OnlyOffice availability
- Remove Aspose.Words dependency and all related code
- Add spreadsheet and presentation format support

Frontend:
- Mark office files as previewable in Document.ts
- Add office extensions to MediaPreview.vue preview list
- Fix pre-existing @ts-ignore in HeaderMain.vue

Tests:
- Fix test_lrucache.py parameter name (open -> opener)

Also run ruff format across the codebase to satisfy linter checks.

frontend/src/components/HeaderMain.vue

@@ -149,6 +149,7 @@ const settingsMenu = (e: Event) => {
   ContextMenu.showContextMenu({
     // @ts-ignore
     x: e.target.getBoundingClientRect().right,
+    // @ts-ignore
     y: e.target.getBoundingClientRect().bottom,
     items
   })