feat: 前端多语言支持（中/英）与品牌 Logo 接入

多语言:
- 新增 vue-i18n，创建 src/locales/zh.ts、en.ts 语言包，按功能模块组织 key
- 新增 src/i18n.ts，语言检测优先级：localStorage > navigator.language（zh→中文，其他→英文）> 兜底英文
- 导航栏新增语言切换按钮（中/EN 一键切换），选择持久化到 localStorage
- 全部 View/Component 硬编码中文替换为 t() 调用，stores/auth.ts 错误消息同样国际化
- index.html lang 属性运行时动态更新

Logo:
- 替换 Vue 默认 logo 为 LmVPN 品牌 Logo（盾牌+网络节点）
- Header 导航栏：Logo 图标 + "LmVPN" 文字
- 首页标题区：大尺寸 Logo 展示
- Favicon 替换为 SVG 格式

frontend/package-lock.json

@@ -10,6 +10,7 @@
       "dependencies": {
         "pinia": "^3.0.4",
         "vue": "^3.5.38",
+        "vue-i18n": "^11.4.6",
         "vue-router": "^5.1.0"
       },
       "devDependencies": {
@@ -518,6 +519,67 @@
         "tslib": "^2.4.0"
       }
     },
+    "node_modules/@intlify/core-base": {
+      "version": "11.4.6",
+      "resolved": "https://registry.npmjs.org/@intlify/core-base/-/core-base-11.4.6.tgz",
+      "integrity": "sha512-EOeHO95XESK9IFHgHeZXunsM/WBAoCA0DlaWODvx14vKmetAuS97t+l6Xe9hTUqntPpF93vtVSjjUDafw3wXMw==",
+      "license": "MIT",
+      "dependencies": {
+        "@intlify/devtools-types": "11.4.6",
+        "@intlify/message-compiler": "11.4.6",
+        "@intlify/shared": "11.4.6"
+      },
+      "engines": {
+        "node": ">= 22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
+    "node_modules/@intlify/devtools-types": {
+      "version": "11.4.6",
+      "resolved": "https://registry.npmjs.org/@intlify/devtools-types/-/devtools-types-11.4.6.tgz",
+      "integrity": "sha512-wowQPpNem56b2d43IJmqbrzG2FeBKe5f/kUGlpNuBmXs6OSqncF8m1+1lxHuW8ISZJF0ma2RkW3iLkw0g0G4VA==",
+      "license": "MIT",
+      "dependencies": {
+        "@intlify/core-base": "11.4.6",
+        "@intlify/shared": "11.4.6"
+      },
+      "engines": {
+        "node": ">= 22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
+    "node_modules/@intlify/message-compiler": {
+      "version": "11.4.6",
+      "resolved": "https://registry.npmjs.org/@intlify/message-compiler/-/message-compiler-11.4.6.tgz",
+      "integrity": "sha512-5nj3jULqeTAC1WovwMs1LQWgatTa2pM/rXN9T3XW8rdOtXW9ZF6/GLSNFTKDQmPLwclhPdgUWLJ/4w3fMeeC/Q==",
+      "license": "MIT",
+      "dependencies": {
+        "@intlify/shared": "11.4.6",
+        "source-map-js": "^1.0.2"
+      },
+      "engines": {
+        "node": ">= 22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
+    "node_modules/@intlify/shared": {
+      "version": "11.4.6",
+      "resolved": "https://registry.npmjs.org/@intlify/shared/-/shared-11.4.6.tgz",
+      "integrity": "sha512-m1p1HHAMLhqSpTRH7VnXdrN0CQ4y+9vunFkpLkbD8soIuBsnQdawZXqMCgvwI2UVF9Ww7sVaw7g9tV2VO7shoA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      }
+    },
     "node_modules/@jridgewell/gen-mapping": {
       "version": "0.3.13",
       "resolved": "https://registry.npmjs.org/@jridgewell/gen-mapping/-/gen-mapping-0.3.13.tgz",
@@ -3343,6 +3405,33 @@
         }
       }
     },
+    "node_modules/vue-i18n": {
+      "version": "11.4.6",
+      "resolved": "https://registry.npmjs.org/vue-i18n/-/vue-i18n-11.4.6.tgz",
+      "integrity": "sha512-l0gE7Rfy0phCa5ChKYkOq543Wgd39BCK6hkktfr1Ed4D99oRkgPK9ffShASZdeC8OJxGfdWmpYoAaAH6iLEuIg==",
+      "license": "MIT",
+      "dependencies": {
+        "@intlify/core-base": "11.4.6",
+        "@intlify/devtools-types": "11.4.6",
+        "@intlify/shared": "11.4.6",
+        "@vue/devtools-api": "^6.5.0"
+      },
+      "engines": {
+        "node": ">= 22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/kazupon"
+      },
+      "peerDependencies": {
+        "vue": "^3.0.0"
+      }
+    },
+    "node_modules/vue-i18n/node_modules/@vue/devtools-api": {
+      "version": "6.6.4",
+      "resolved": "https://registry.npmjs.org/@vue/devtools-api/-/devtools-api-6.6.4.tgz",
+      "integrity": "sha512-sGhTPMuXqZ1rVOk32RylztWkfXTRhuS7vgAKv0zjqk8gbsHkJ7xfFf+jbySxt7tWObEJwyKaHMikV/WGDiQm8g==",
+      "license": "MIT"
+    },
     "node_modules/vue-router": {
       "version": "5.1.0",
       "resolved": "https://registry.npmjs.org/vue-router/-/vue-router-5.1.0.tgz",