Add first-visit guard

Contributes to #28

src/app/app-routing.module.ts

@@ -1,14 +1,20 @@
 import { NgModule } from '@angular/core';
 import { PreloadAllModules, RouterModule, Routes } from '@angular/router';
 
+import { FirstVisitGuard } from './shared/first-visit.guard';
+
 const routes: Routes = [
   {
     path: '',
-    loadChildren: () => import('./tabs/tabs.module').then(m => m.TabsPageModule)
+    loadChildren: () =>
+      import('./tabs/tabs.module').then(m => m.TabsPageModule),
+    canActivate: [FirstVisitGuard]
   },
   {
     path: 'auth',
-    loadChildren: () => import('./auth/auth.module').then(m => m.AuthPageModule)
+    loadChildren: () =>
+      import('./auth/auth.module').then(m => m.AuthPageModule),
+    canActivate: [FirstVisitGuard]
   }
 ];
 @NgModule({

src/app/shared/first-visit.guard.ts

@@ -0,0 +1,30 @@
+import { Injectable } from '@angular/core';
+import {
+  ActivatedRouteSnapshot,
+  RouterStateSnapshot,
+  UrlTree,
+  CanActivate,
+  Router
+} from '@angular/router';
+import { Plugins } from '@capacitor/core';
+import { Observable } from 'rxjs';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class FirstVisitGuard implements CanActivate {
+  constructor(private router: Router) {}
+
+  canActivate(
+    route: ActivatedRouteSnapshot,
+    state: RouterStateSnapshot
+  ):
+    | boolean
+    | UrlTree
+    | Observable<boolean | UrlTree>
+    | Promise<boolean | UrlTree> {
+    return Plugins.Storage.get({ key: 'visited' }).then(value =>
+      !!value.value ? true : this.router.createUrlTree(['welcome'])
+    );
+  }
+}