feat: add DOCX/XLSX/PPTX file preview

- DOCX: mammoth converts to semantic HTML (prose preview)
- XLSX: xlsx library extended to FileNav with sheet tabs at bottom
- PPTX: custom canvas renderer produces PNG images per slide
  with panzoom zoom/pan and slide navigation

Changes:
- New: src/lib/utils/pptxToHtml.ts (canvas-based PPTX renderer)
- FileNav.svelte: office format detection, blob download, conversion
- FilePreview.svelte: office rendering branches, sheet tabs, slide viewer
- FileItemModal.svelte: DOCX/PPTX preview tabs
- package.json: added mammoth dependency

package.json

@@ -110,6 +110,7 @@
 		"idb": "^7.1.1",
 		"js-sha256": "^0.10.1",
 		"jspdf": "^4.0.0",
+		"jszip": "^3.10.1",
 		"katex": "^0.16.22",
 		"kokoro-js": "^1.1.1",
 		"leaflet": "^1.9.4",