feat: 配方查询支持按精油名搜索

输入精油中文名/英文名会返回含该精油的所有配方。
中文查询 ≥2 字才匹配精油，避免「草」这样的单字噪音。

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

frontend/cypress/e2e/recipe-search.cy.js

@@ -26,6 +26,14 @@ describe('Recipe Search', () => {
     })
   })
 
+  it('searching by oil name returns recipes containing that oil', () => {
+    cy.get('input[placeholder*="搜索"]').type('薰衣草')
+    cy.wait(500)
+    cy.get('.search-results-section, .recipe-card', { timeout: 5000 }).should('exist')
+    // At least one result card should exist (any recipe using 薰衣草)
+    cy.get('.recipe-card').should('have.length.gte', 1)
+  })
+
   it('clears search and restores all recipes', () => {
     cy.get('.recipe-card', { timeout: 10000 }).should('have.length.gte', 1)
     cy.get('input[placeholder*="搜索"]').type('薰衣草')