feat(admin): max uses column + used count in the promo editor

The Quasar editor gains a "Max uses" input per code (blank = unlimited,
normalised to null on save) and shows "Used n / max" from the derived
count. The public buy page's Clear button now also clears the promo field.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ByAwHU4pRnyE58YocQvAas

static/js/display.js

@@ -94,6 +94,7 @@ window.PageEventsDisplay = {
       this.formDialog.data.email = ''
       this.formDialog.data.refund = ''
       this.formDialog.data.nostr_identifier = ''
+      this.formDialog.data.promo_code = ''
       this.formDialog.data.payment_method =
         this.paymentMethods[0] || 'lightning'
     },