[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/platform/workspace/auth/WorkspaceAuthGate.vue

@@ -4,7 +4,7 @@
     v-else
     class="fixed inset-0 z-[1100] flex items-center justify-center bg-[var(--p-mask-background)]"
   >
-    <ProgressSpinner />
+    <Loader size="lg" class="text-white" />
   </div>
 </template>
 
@@ -22,8 +22,8 @@
  * instead of workspace tokens when the workspace feature is enabled.
  */
 import { promiseTimeout, until } from '@vueuse/core'
+import Loader from '@/components/common/Loader.vue'
 import { storeToRefs } from 'pinia'
-import ProgressSpinner from 'primevue/progressspinner'
 import { onMounted, ref } from 'vue'
 
 import { useFeatureFlags } from '@/composables/useFeatureFlags'