feat: 增加 i18n 能力（独立 vue-i18n 实例 + useLocale + 中英 locale）

- 新增 src/i18n/（独立实例，中文同步、英文懒加载）+ src/composables/useLocale.js
- 抽取所有硬编码中文为 locale 文案（zh-CN/en）
- 组件改用 useLocale 直读自身 i18n 实例（嵌入时不走 useI18n 注入）
- App.vue 接 :locale prop，watch 同步父项目语言（嵌入 mainPage 时桥接）
- 42 项测试全通过，构建正常

package-lock.json

@@ -9,7 +9,8 @@
       "version": "0.1.0",
       "dependencies": {
         "ical.js": "^2.1.0",
-        "vue": "^3.5.0"
+        "vue": "^3.5.0",
+        "vue-i18n": "^9.13.1"
       },
       "devDependencies": {
         "@vitejs/plugin-vue": "^5.2.0",
@@ -505,6 +506,47 @@
         "node": ">=18"
       }
     },
+    "node_modules/@intlify/core-base": {
+      "version": "9.14.4",
+      "resolved": "https://registry.npmmirror.com/@intlify/core-base/-/core-base-9.14.4.tgz",
+      "integrity": "sha512-vtZCt7NqWhKEtHa3SD/322DlgP5uR9MqWxnE0y8Q0tjDs9H5Lxhss+b5wv8rmuXRoHKLESNgw9d+EN9ybBbj9g==",
+      "dependencies": {
+        "@intlify/message-compiler": "9.14.4",
+        "@intlify/shared": "9.14.4"
+      },
+      "engines": {
+        "node": ">= 16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
+    "node_modules/@intlify/message-compiler": {
+      "version": "9.14.4",
+      "resolved": "https://registry.npmmirror.com/@intlify/message-compiler/-/message-compiler-9.14.4.tgz",
+      "integrity": "sha512-vcyCLiVRN628U38c3PbahrhbbXrckrM9zpy0KZVlDk2Z0OnGwv8uQNNXP3twwGtfLsCf4gu3ci6FMIZnPaqZsw==",
+      "dependencies": {
+        "@intlify/shared": "9.14.4",
+        "source-map-js": "^1.0.2"
+      },
+      "engines": {
+        "node": ">= 16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
+    "node_modules/@intlify/shared": {
+      "version": "9.14.4",
+      "resolved": "https://registry.npmmirror.com/@intlify/shared/-/shared-9.14.4.tgz",
+      "integrity": "sha512-P9zv6i1WvMc9qDBWvIgKkymjY2ptIiQ065PjDv7z7fDqH3J/HBRBN5IoiR46r/ujRcU7hCuSIZWvCAFCyuOYZA==",
+      "engines": {
+        "node": ">= 16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
     "node_modules/@jridgewell/sourcemap-codec": {
       "version": "1.5.5",
       "resolved": "https://registry.npmmirror.com/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.5.5.tgz",
@@ -1018,6 +1060,11 @@
         "@vue/shared": "3.5.39"
       }
     },
+    "node_modules/@vue/devtools-api": {
+      "version": "6.6.4",
+      "resolved": "https://registry.npmmirror.com/@vue/devtools-api/-/devtools-api-6.6.4.tgz",
+      "integrity": "sha512-sGhTPMuXqZ1rVOk32RylztWkfXTRhuS7vgAKv0zjqk8gbsHkJ7xfFf+jbySxt7tWObEJwyKaHMikV/WGDiQm8g=="
+    },
     "node_modules/@vue/reactivity": {
       "version": "3.5.39",
       "resolved": "https://registry.npmmirror.com/@vue/reactivity/-/reactivity-3.5.39.tgz",
@@ -2708,6 +2755,25 @@
         }
       }
     },
+    "node_modules/vue-i18n": {
+      "version": "9.14.4",
+      "resolved": "https://registry.npmmirror.com/vue-i18n/-/vue-i18n-9.14.4.tgz",
+      "integrity": "sha512-B934C8yUyWLT0EMud3DySrwSUJI7ZNiWYsEEz2gknTthqKiG4dzWE/WSa8AzCuSQzwBEv4HtG1jZDhgzPfWSKQ==",
+      "dependencies": {
+        "@intlify/core-base": "9.14.4",
+        "@intlify/shared": "9.14.4",
+        "@vue/devtools-api": "^6.5.0"
+      },
+      "engines": {
+        "node": ">= 16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      },
+      "peerDependencies": {
+        "vue": "^3.0.0"
+      }
+    },
     "node_modules/why-is-node-running": {
       "version": "2.3.0",
       "resolved": "https://registry.npmmirror.com/why-is-node-running/-/why-is-node-running-2.3.0.tgz",