[refactor] Replace PrimeVue ProgressSpinner with Lucide loader icon (#9372)

## Summary
- Replace PrimeVue `ProgressSpinner` with Lucide `loader-circle` icon in
App.vue and WorkspaceAuthGate.vue
- Use white color for loading spinner for better visibility on dark
backgrounds
- Remove `primevue/progressspinner` imports and update related test

## Changes
- **App.vue**: Replace `ProgressSpinner` with
`icon-[lucide--loader-circle]`
- **WorkspaceAuthGate.vue**: Same replacement
- **WorkspaceAuthGate.test.ts**: Remove ProgressSpinner mock, use
`.animate-spin` selector

## Review Focus
- Visual consistency of white spinner on dark background during initial
load

<img width="1596" height="1189" alt="스크린샷 2026-03-04 오후 6 28 27"
src="https://github.com/user-attachments/assets/d703db74-4123-4328-912a-45ac45cf6eeb"
/>
<img width="1680" height="1304" alt="스크린샷 2026-03-04 오후 6 28 24"
src="https://github.com/user-attachments/assets/8026d10a-7e06-4f95-849c-bc891756823c"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9372-refactor-Replace-PrimeVue-ProgressSpinner-with-Lucide-loader-icon-3196d73d3650815bb1d1d4554f7f744e)
by [Unito](https://www.unito.io)

src/App.vue

@@ -1,9 +1,11 @@
 <template>
   <router-view />
-  <ProgressSpinner
+  <div
     v-if="isLoading"
-    class="absolute inset-0 flex h-[unset] items-center justify-center"
-  />
+    class="absolute inset-0 flex items-center justify-center"
+  >
+    <Loader size="lg" class="text-white" />
+  </div>
   <GlobalDialog />
   <BlockUI full-screen :blocked="isLoading" />
 </template>
@@ -11,9 +13,9 @@
 <script setup lang="ts">
 import { captureException } from '@sentry/vue'
 import BlockUI from 'primevue/blockui'
-import ProgressSpinner from 'primevue/progressspinner'
 import { computed, onMounted } from 'vue'
 
+import Loader from '@/components/common/Loader.vue'
 import GlobalDialog from '@/components/dialog/GlobalDialog.vue'
 import config from '@/config'
 import { useWorkspaceStore } from '@/stores/workspaceStore'