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/login.vue

@@ -3,6 +3,7 @@ import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card'
 import { Input } from '@/components/ui/input'
 import { Label } from '@/components/ui/label'
+import { Spinner } from '@/components/ui/spinner'
 
 const { login } = useAuth()
 const { t } = useI18n()
@@ -50,6 +51,7 @@ async function onSubmit() {
             {{ errorMessage }}
           </p>
           <Button type="submit" class="w-full" :disabled="loading">
+            <Spinner v-if="loading" />
             {{ $t('auth.login') }}
           </Button>
           <p class="text-center text-sm text-muted-foreground">