feat(loyalty): make logo URL mandatory on program edit forms

Logo URL is required by Google Wallet API for LoyaltyClass creation.
Added validation across all three program edit screens (admin, merchant, store)
with a helpful hint explaining the requirement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/loyalty/static/admin/js/loyalty-program-edit.js

@@ -100,6 +100,7 @@ function adminLoyaltyProgramEdit() {
 
             try {
                 const payload = this.buildPayload();
+                if (!payload) { this.saving = false; return; }
 
                 if (this.isNewProgram) {
                     const response = await apiClient.post(