fix: mobile modal UX improvements (delete confirm, browser-back, z-index) (#206)

- Replace browser confirm() with ConfirmModal for delete confirmation
- Add dedicated history entry for delete dialog so browser back dismisses it
- Track unsaved-changes warning source to restore correct context on cancel
- Add overlayClassName prop to ConfirmModal for nested z-index layering
- Add .nested-confirm CSS class for proper modal stacking
- Add i18n keys for delete confirmation dialog (EN + DE)

Closes #202

frontend/src/i18n/de.json

@@ -136,6 +136,10 @@
       "stock": "Bestand",
       "totalCapacity": "Kapazität",
       "type": "Typ"
+    },
+    "deleteModal": {
+      "title": "Medikament löschen",
+      "message": "Möchtest du \"{{name}}\" wirklich löschen?"
     }
   },
   "form": {

frontend/src/i18n/en.json

@@ -136,6 +136,10 @@
       "stock": "Stock",
       "totalCapacity": "Capacity",
       "type": "Type"
+    },
+    "deleteModal": {
+      "title": "Delete medication",
+      "message": "Do you really want to delete \"{{name}}\"?"
     }
   },
   "form": {