feat(01-06): add plan card component and switch action

- Create switchPlan server action for plan changes
- Create getCurrentPlan utility function
- Build PlanCard component with feature display
- Handle plan switching with loading state
- Revalidate dashboard and subscription pages on change

src/app/actions/subscription.ts

@@ -0,0 +1,80 @@
+'use server'
+
+import { createClient } from '@/lib/supabase/server'
+import { revalidatePath } from 'next/cache'
+
+export type SubscriptionActionState = {
+  success?: boolean
+  error?: string
+  message?: string
+}
+
+export async function switchPlan(planId: string): Promise<SubscriptionActionState> {
+  const supabase = await createClient()
+
+  // Get current user
+  const { data: { user }, error: authError } = await supabase.auth.getUser()
+
+  if (authError || !user) {
+    return { error: 'Devi essere autenticato per cambiare piano' }
+  }
+
+  // Verify plan exists
+  const { data: plan, error: planError } = await supabase
+    .from('plans')
+    .select('id, name, display_name_it')
+    .eq('id', planId)
+    .single()
+
+  if (planError || !plan) {
+    return { error: 'Piano non trovato' }
+  }
+
+  // Update user's plan
+  const { error: updateError } = await supabase
+    .from('profiles')
+    .update({ plan_id: planId })
+    .eq('id', user.id)
+
+  if (updateError) {
+    console.error('Failed to update plan:', updateError)
+    return { error: 'Errore durante il cambio piano. Riprova.' }
+  }
+
+  // Revalidate pages that show plan info
+  revalidatePath('/dashboard')
+  revalidatePath('/subscription')
+
+  return {
+    success: true,
+    message: `Piano cambiato a ${plan.display_name_it}`
+  }
+}
+
+export async function getCurrentPlan() {
+  const supabase = await createClient()
+
+  const { data: { user } } = await supabase.auth.getUser()
+
+  if (!user) {
+    return null
+  }
+
+  const { data: profile } = await supabase
+    .from('profiles')
+    .select(`
+      plan_id,
+      plans (
+        id,
+        name,
+        display_name,
+        display_name_it,
+        price_monthly,
+        features
+      )
+    `)
+    .eq('id', user.id)
+    .single()
+
+  return profile?.plans || null
+}