fix: PDF rendering in production builds and XLSX formula calculation

PDF: Replace iframe-based PDF rendering with PDF.js canvas rendering.
WebKitGTK (Linux production builds) has no built-in PDF viewer, so
iframes silently failed. Now renders PDFs via pdfjs-dist on all platforms.

XLSX: Add HyperFormula engine and enable sheetStubs for formula cells
with empty cached values. Files generated without pre-computed results
now have their formulas evaluated at display time.

Author: derekross

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "onyx",
-  "version": "0.14.4",
+  "version": "0.14.5",
   "description": "Open source knowledge base and note-taking app",
   "type": "module",
   "scripts": {
@@ -45,9 +45,11 @@
     "@xterm/xterm": "^5.5.0",
     "codemirror": "^6.0.2",
     "dayjs": "^1.11.19",
+    "hyperformula": "^3.2.0",
     "jszip": "^3.10.1",
     "mammoth": "^1.11.0",
     "nostr-tools": "^2.19.4",
+    "pdfjs-dist": "^5.5.207",
     "qrcode": "^1.5.4",
     "qrcode.react": "^4.2.0",
     "solid-js": "^1.9.10",