feat: user frontend i18n (zh-CN/en)

- vue-i18n@9 with locale files for zh-CN and en
- Language switcher in topbar
- Navigation, breadcrumb, credits page translated
- Discovery page i18n keys prepared
- Language persisted in localStorage
- Build verified

user-frontend/package-lock.json

@@ -14,6 +14,7 @@
         "element-plus": "^2.9.1",
         "pinia": "^2.3.0",
         "vue": "^3.5.13",
+        "vue-i18n": "^9.14.4",
         "vue-router": "^4.5.0"
       },
       "devDependencies": {
@@ -553,6 +554,50 @@
       "integrity": "sha512-RiB/yIh78pcIxl6lLMG0CgBXAZ2Y0eVHqMPYugu+9U0AeT6YBeiJpf7lbdJNIugFP5SIjwNRgo4DhR1Qxi26Gg==",
       "license": "MIT"
     },
+    "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==",
+      "license": "MIT",
+      "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==",
+      "license": "MIT",
+      "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==",
+      "license": "MIT",
+      "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",
@@ -1990,6 +2035,26 @@
         }
       }
     },
+    "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==",
+      "license": "MIT",
+      "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/vue-router": {
       "version": "4.6.4",
       "resolved": "https://registry.npmmirror.com/vue-router/-/vue-router-4.6.4.tgz",