feat(vendor): add frontend for contact info inheritance

Add UI for vendor contact field inheritance from company:
- Show "(from company)" indicator for inherited fields
- Add "Reset" button per field to clear override
- Add "Reset All to Company" button for bulk reset
- Purple border styling for inherited fields
- Dynamic placeholder showing company values

JavaScript methods:
- resetFieldToCompany(fieldName): Reset individual field
- resetAllContactToCompany(): Reset all contact fields
- hasAnyContactOverride(): Check if any field is overridden

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/admin/js/vendor-edit.js

@@ -254,6 +254,57 @@ function adminVendorEdit() {
             } finally {
                 this.saving = false;
             }
+        },
+
+        // ===== Contact Field Inheritance Methods =====
+
+        /**
+         * Reset a single contact field to inherit from company.
+         * Sets the field to empty string, which the backend converts to null (inherit).
+         * @param {string} fieldName - The contact field to reset
+         */
+        resetFieldToCompany(fieldName) {
+            const contactFields = ['contact_email', 'contact_phone', 'website', 'business_address', 'tax_number'];
+            if (!contactFields.includes(fieldName)) {
+                editLog.warn('Invalid contact field:', fieldName);
+                return;
+            }
+
+            editLog.info(`Resetting ${fieldName} to inherit from company`);
+            this.formData[fieldName] = '';
+
+            // Update the vendor object to reflect inheritance (UI indicator)
+            if (this.vendor) {
+                this.vendor[`${fieldName}_inherited`] = true;
+            }
+        },
+
+        /**
+         * Reset all contact fields to inherit from company.
+         */
+        resetAllContactToCompany() {
+            editLog.info('Resetting all contact fields to inherit from company');
+
+            const contactFields = ['contact_email', 'contact_phone', 'website', 'business_address', 'tax_number'];
+            contactFields.forEach(field => {
+                this.formData[field] = '';
+                if (this.vendor) {
+                    this.vendor[`${field}_inherited`] = true;
+                }
+            });
+
+            Utils.showToast('All contact fields reset to company defaults', 'info');
+        },
+
+        /**
+         * Check if any contact field has a vendor-level override (not inherited).
+         * @returns {boolean} True if at least one contact field is overridden
+         */
+        hasAnyContactOverride() {
+            if (!this.vendor) return false;
+
+            const contactFields = ['contact_email', 'contact_phone', 'website', 'business_address', 'tax_number'];
+            return contactFields.some(field => !this.vendor[`${field}_inherited`]);
         }
     };
 }