Expand test suite to 364 tests (168 unit + 196 E2E)

Unit tests:
- Volume/dilution calculation (63 tests): scaling, mode detection,
  ratio calculation, real recipe round-trip verification

E2E tests:
- Batch operations: create/tag/delete 3 recipes, adopt workflow
- Projects: CRUD, pricing, profit calculation vs oil costs
- Notifications: fetch, fields, mark-all-read
- Account settings: profile read/update, auth rejection
- Category modules: listing, tag reference
- Registration: register, login, duplicate rejection
- Audit log: pagination, field validation, action tracking

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

frontend/cypress/e2e/notification-flow.cy.js

@@ -0,0 +1,38 @@
+describe('Notification Flow', () => {
+  const ADMIN_TOKEN = 'c86ae7afbe10fabe3c1d5e1a7fee74feaadfd5dc7be2ab62'
+  const authHeaders = { Authorization: `Bearer ${ADMIN_TOKEN}` }
+
+  it('fetches notifications', () => {
+    cy.request({ url: '/api/notifications', headers: authHeaders }).then(res => {
+      expect(res.status).to.eq(200)
+      expect(res.body).to.be.an('array')
+    })
+  })
+
+  it('each notification has required fields', () => {
+    cy.request({ url: '/api/notifications', headers: authHeaders }).then(res => {
+      if (res.body.length > 0) {
+        const n = res.body[0]
+        expect(n).to.have.property('title')
+        expect(n).to.have.property('is_read')
+        expect(n).to.have.property('created_at')
+      }
+    })
+  })
+
+  it('can mark all notifications as read', () => {
+    cy.request({
+      method: 'POST', url: '/api/notifications/read-all',
+      headers: authHeaders, body: {}
+    }).then(res => {
+      expect(res.status).to.eq(200)
+    })
+  })
+
+  it('all notifications are now read', () => {
+    cy.request({ url: '/api/notifications', headers: authHeaders }).then(res => {
+      const unread = res.body.filter(n => !n.is_read)
+      expect(unread).to.have.length(0)
+    })
+  })
+})