refactor(settings): split Advanced tab into Proxy tab and move Pricing to Usage

Extract proxy-related accordion items (Local Proxy, Failover, Rectifier,
Global Outbound Proxy) into a dedicated Proxy tab via ProxyTabContent
component. Move Pricing config panel to UsageDashboard as a collapsible
accordion. This reduces SettingsPage from ~716 to ~426 lines and improves
settings discoverability with a 5-tab layout: General | Proxy | Advanced |
Usage | About.

src/i18n/locales/en.json

@@ -187,6 +187,7 @@
     "general": "General",
     "tabGeneral": "General",
     "tabAdvanced": "Advanced",
+    "tabProxy": "Proxy",
     "advanced": {
       "configDir": {
         "title": "Configuration Directory",

src/i18n/locales/ja.json

@@ -187,6 +187,7 @@
     "general": "一般",
     "tabGeneral": "一般",
     "tabAdvanced": "詳細",
+    "tabProxy": "プロキシ",
     "advanced": {
       "configDir": {
         "title": "設定ディレクトリ",

src/i18n/locales/zh.json

@@ -187,6 +187,7 @@
     "general": "通用",
     "tabGeneral": "通用",
     "tabAdvanced": "高级",
+    "tabProxy": "代理",
     "advanced": {
       "configDir": {
         "title": "配置文件目录",