Shared Spinner component for button loading states

Adds a reusable shadcn-style Spinner (lucide loader-circle icon,
localized aria-label) and wires it into every button that has a
pending state: comment submit, delete, tag save, profile save,
admin saves, upload, login and register.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/pages/settings.vue

@@ -33,6 +33,7 @@ import {
   SelectTrigger,
   SelectValue,
 } from '@/components/ui/select'
+import { Spinner } from '@/components/ui/spinner'
 
 const { user, refresh } = useAuth()
 const { invalidate } = useUserSummaries()
@@ -268,6 +269,7 @@ async function save() {
             <FileDropzone v-model="avatar" accept="image/*" :hint="t('upload.avatarHint')" />
           </div>
           <Button type="submit" class="w-full" :disabled="saving">
+            <Spinner v-if="saving" />
             {{ saving ? $t('common.saving') : $t('common.save') }}
           </Button>
         </form>