feat: pick username

* put new registered social login user to session data
* ask to choose username
* simplify checkUsername functions

app/src/views/ChooseUsernamePage.vue

@@ -0,0 +1,16 @@
+<script setup>
+import UsernameForm from '../components/UsernameForm.vue'
+import Background from '../components/DynamicBackground.vue'
+</script>
+<template>
+  <div class="relative w-screen h-screen">
+    <Background class="w-full h-full" />
+    <div
+      class="h-screen w-screen absolute top-0 left-0 flex flex-wrap justify-center content-center"
+    >
+      <div class="h-2/3 w-1/3">
+        <UsernameForm />
+      </div>
+    </div>
+  </div>
+</template>