Refactor frontend to Vue 3 + Vite + Pinia + Cypress E2E

- Replace single-file 8441-line HTML with Vue 3 SPA
- Pinia stores: auth, oils, recipes, diary, ui
- Composables: useApi, useDialog, useSmartPaste, useOilTranslation
- 6 shared components: RecipeCard, RecipeDetailOverlay, TagPicker, etc.
- 9 page views: RecipeSearch, RecipeManager, Inventory, OilReference, etc.
- 14 Cypress E2E test specs (113 tests), all passing
- Multi-stage Dockerfile (Node build + Python runtime)
- Demo video generation scripts (TTS + subtitles + screen recording)

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

frontend/src/stores/ui.js

@@ -0,0 +1,40 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+export const useUiStore = defineStore('ui', () => {
+  const currentSection = ref('search')
+  const showLoginModal = ref(false)
+  const toasts = ref([])
+
+  let toastId = 0
+
+  function showSection(name) {
+    currentSection.value = name
+  }
+
+  function showToast(msg, duration = 1800) {
+    const id = ++toastId
+    toasts.value.push({ id, msg })
+    setTimeout(() => {
+      toasts.value = toasts.value.filter((t) => t.id !== id)
+    }, duration)
+  }
+
+  function openLogin() {
+    showLoginModal.value = true
+  }
+
+  function closeLogin() {
+    showLoginModal.value = false
+  }
+
+  return {
+    currentSection,
+    showLoginModal,
+    toasts,
+    showSection,
+    showToast,
+    openLogin,
+    closeLogin,
+  }
+})