feat: 精油价目导出改为 Excel（.xlsx）

- 按钮从「导出PDF」改「导出Excel」，动态 import xlsx
- 列：精油/英文名/会员价/零售价/容量/单价/状态
- 文件名：精油价目表YYYY-MM-DD.xlsx
- 新增 e2e（按钮可见 + 点击出 toast）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/cypress/e2e/oil-price-export.cy.js

@@ -0,0 +1,28 @@
+describe('Oil Reference Excel Export', () => {
+  let adminToken
+
+  before(() => {
+    cy.getAdminToken().then(token => { adminToken = token })
+  })
+
+  beforeEach(() => {
+    cy.visit('/oils', {
+      onBeforeLoad(win) {
+        win.localStorage.setItem('oil_auth_token', adminToken)
+      }
+    })
+    cy.get('.oil-chip, .oils-grid', { timeout: 10000 }).should('exist')
+  })
+
+  it('shows the Excel export button for admins', () => {
+    cy.contains('button', '📥 导出Excel').should('be.visible')
+  })
+
+  it('clicking Excel export shows success toast', () => {
+    cy.window().then(win => {
+      cy.stub(win.HTMLAnchorElement.prototype, 'click').returns(undefined)
+    })
+    cy.contains('button', '📥 导出Excel').click()
+    cy.get('.toast', { timeout: 10000 }).should('contain', '导出成功')
+  })
+})