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'
     },

static/js/index.js

@@ -526,7 +526,9 @@ window.PageEvents = {
           .filter(code => code.code?.trim() !== '')
           .map(code => ({
             ...code,
-            code: code.code.trim().toUpperCase()
+            code: code.code.trim().toUpperCase(),
+            // blank / 0 = unlimited; used_count is derived server-side
+            max_uses: code.max_uses ? Number(code.max_uses) : null
           }))
       }
       const methods = data.extra?.payment_methods || []

static/js/index.vue

@@ -729,12 +729,22 @@
               class="row q-col-gutter-sm q-mt-md"
             >
               <q-input
-                class="col-8"
+                class="col-6"
                 filled
                 dense
                 v-model.trim="formDialog.data.extra.promo_codes[index].code"
                 type="text"
                 label="Promo Code"
+                :hint="
+                  formDialog.data.extra.promo_codes[index].used_count
+                    ? 'Used ' +
+                      formDialog.data.extra.promo_codes[index].used_count +
+                      (formDialog.data.extra.promo_codes[index].max_uses
+                        ? ' / ' +
+                          formDialog.data.extra.promo_codes[index].max_uses
+                        : '')
+                    : ''
+                "
               >
                 <template v-slot:before>
                   <q-checkbox
@@ -755,7 +765,7 @@
                 </template>
               </q-input>
               <q-input
-                class="col-4"
+                class="col-3"
                 filled
                 dense
                 v-model.number="
@@ -765,6 +775,19 @@
                 label="Discount (%)"
                 min="0"
                 max="100"
+              ></q-input>
+              <q-input
+                class="col-3"
+                filled
+                dense
+                v-model.number="
+                  formDialog.data.extra.promo_codes[index].max_uses
+                "
+                type="number"
+                label="Max uses"
+                placeholder="∞"
+                min="1"
+                hint="Blank = unlimited"
               >
                 <template v-slot:after>
                   <q-btn
@@ -783,7 +806,8 @@
                   formDialog.data.extra.promo_codes.push({
                     code: '',
                     discount_percent: 0,
-                    active: true
+                    active: true,
+                    max_uses: null
                   })
                 "
                 >Add Promo Code</q-btn