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/components/UploadPopover.vue

@@ -11,6 +11,7 @@ import {
   PopoverTitle,
   PopoverTrigger,
 } from '@/components/ui/popover'
+import { Spinner } from '@/components/ui/spinner'
 
 const { user } = useAuth()
 const { t } = useI18n()
@@ -82,6 +83,7 @@ async function onSubmit() {
             {{ errorMessage }}
           </p>
           <Button type="submit" class="w-full" :disabled="loading || (!file && !url.trim())">
+            <Spinner v-if="loading" />
             {{ loading ? $t('upload.uploading') : $t('upload.upload') }}
           </Button>
         </form>