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>

src/components/SheetCanvas.vue

@@ -133,7 +133,7 @@ function selectObj(id: string) {
       </div>
     </div>
 
-    <div v-if="dropActive" class="drop">Datei ablegen (.json .csv .xlsx)</div>
+    <div v-if="dropActive" class="drop">Datei ablegen (.num .json .csv .xlsx)</div>
   </div>
 </template>