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-lock.json

@@ -9,6 +9,7 @@
       "version": "1.0.0",
       "dependencies": {
         "chart.js": "^4.4.6",
+        "fflate": "^0.8.3",
         "vue": "^3.5.13",
         "xlsx": "^0.18.5"
       },
@@ -2952,6 +2953,12 @@
         "reusify": "^1.0.4"
       }
     },
+    "node_modules/fflate": {
+      "version": "0.8.3",
+      "resolved": "https://registry.npmjs.org/fflate/-/fflate-0.8.3.tgz",
+      "integrity": "sha512-tbZNuJrLwGUp3zshBtdy4W+ORxZuIh8a5ilyIEQDC5rY1f3U20JMry0Ll3WBzU58EZKsEuJFXhb5gwv8CsPvgA==",
+      "license": "MIT"
+    },
     "node_modules/file-entry-cache": {
       "version": "8.0.0",
       "resolved": "https://registry.npmjs.org/file-entry-cache/-/file-entry-cache-8.0.0.tgz",