Add offline cachine for file group elements and contents (not files, though)

src/app/components/option-picker/option-picker.component.ts

@@ -2,6 +2,7 @@ import {Component, Input, OnInit} from '@angular/core';
 import {Router} from '@angular/router';
 import {ApiService} from '../../service/api.service';
 import {PopoverController} from '@ionic/angular';
+import {DatabaseService} from "../../service/db/database.service";
 
 @Component({
   selector: 'app-option-picker',
@@ -17,14 +18,16 @@ export class OptionPickerComponent implements OnInit {
       protected api: ApiService,
       protected router: Router,
       protected popover: PopoverController,
+      protected db: DatabaseService,
   ) { }
 
   ngOnInit() {}
 
-  onSelect(key) {
+  async onSelect(key) {
     if ( key === 'html_export' ) {
       window.open(this.api._build_url('/data/export/html'), '_blank');
     } else if ( key === 'logout' ) {
+      await this.db.purge();
       window.location.href = '/auth/logout';
     } else if ( key === 'toggle_darkmode' ) {
       this.toggleDark();