Delete invoice feature (#64)

* I mplement delete API for draft invoices with ownership verification

* Color change dark mode

app/api/invoices/[id]/route.ts

@@ -0,0 +1,64 @@
+import { createClient } from '@/lib/supabase/server'
+import { NextResponse } from 'next/server'
+
+/**
+ * DELETE /api/invoices/[id]
+ *
+ * Permanently deletes a draft invoice and its items.
+ * Only invoices with status 'draft' can be deleted — committed invoices
+ * are immutable per BFL and must be reversed via credit note instead.
+ */
+export async function DELETE(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const { id } = await params
+  const supabase = await createClient()
+
+  const { data: { user } } = await supabase.auth.getUser()
+
+  if (!user) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  // Fetch invoice to verify ownership and status
+  const { data: invoice, error: fetchError } = await supabase
+    .from('invoices')
+    .select('id, status, user_id')
+    .eq('id', id)
+    .eq('user_id', user.id)
+    .single()
+
+  if (fetchError || !invoice) {
+    return NextResponse.json({ error: 'Invoice not found' }, { status: 404 })
+  }
+
+  if (invoice.status !== 'draft') {
+    return NextResponse.json(
+      { error: 'Endast utkast kan tas bort. Bokförda fakturor måste krediteras istället.' },
+      { status: 400 }
+    )
+  }
+
+  // Delete items first (FK constraint), then the invoice
+  const { error: itemsError } = await supabase
+    .from('invoice_items')
+    .delete()
+    .eq('invoice_id', id)
+
+  if (itemsError) {
+    return NextResponse.json({ error: itemsError.message }, { status: 500 })
+  }
+
+  const { error: deleteError } = await supabase
+    .from('invoices')
+    .delete()
+    .eq('id', id)
+    .eq('user_id', user.id)
+
+  if (deleteError) {
+    return NextResponse.json({ error: deleteError.message }, { status: 500 })
+  }
+
+  return NextResponse.json({ data: { deleted: true } })
+}