Save and open documents as .num (zipped JSON)

Datei › Speichern writes a .num file: a ZIP (fflate) laid out like
OpenDocument -- an uncompressed "mimetype" entry first
(application/x-numolus), meta.json (format version, ISO 8601 creation
time) and document.json, the same JSON the .json export writes. Opening a
.num (file dialog or drag & drop) unpacks it and offers the same
replace/append choice as a .json. Non-ZIPs, foreign ZIPs, a missing
document.json and a newer format version fail with a clear message.

The zip code is loaded on first use, like xlsx, so the main bundle does
not grow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "chart.js": "^4.4.6",
+    "fflate": "^0.8.3",
     "vue": "^3.5.13",
     "xlsx": "^0.18.5"
   },
@@ -32,4 +33,4 @@
     "vitest": "^2.1.5",
     "vue-tsc": "^2.1.10"
   }
-}
+}