refactor(claude-desktop): align provider form UI with Claude Code

- Rename field labels: "Gateway Base URL" → "API Endpoint", "Bearer Token" → "API Key"
- Change layout from 2-column grid to vertical sections matching Claude Code
- Reuse shared EndpointField component with format-aware amber hint box
- Replace native <datalist> with vendor-grouped ModelDropdown (OpenCode pattern)
- Move Fetch/Add buttons to section headers with compact sm styling
- Extract ModelDropdown to shared module, deduplicate from OpenCodeFormFields
- Extract renderActionButtons helper to eliminate proxy/direct button duplication
- Remove dead i18n keys (gatewayBaseUrl, bearerToken) from all 3 locales

src/i18n/locales/en.json

@@ -177,8 +177,6 @@
     "mode": "Model handling",
     "modeDirect": "Direct",
     "modeProxy": "Model mapping",
-    "gatewayBaseUrl": "Gateway Base URL",
-    "bearerToken": "Bearer Token",
     "modelMappingToggle": "Needs model mapping",
     "modelMappingOffHint": "Use this when the provider already exposes and accepts claude-* / anthropic/claude-* model IDs through Anthropic Messages. Claude Desktop connects to the provider directly.",
     "modelMappingOnHint": "Claude Desktop only sees claude-* route names. CC Switch local routing maps them to the real upstream models, so local routing must stay running.",

src/i18n/locales/ja.json

@@ -177,8 +177,6 @@
     "mode": "モデル処理方式",
     "modeDirect": "直結",
     "modeProxy": "モデルマッピング",
-    "gatewayBaseUrl": "Gateway Base URL",
-    "bearerToken": "Bearer Token",
     "modelMappingToggle": "モデルマッピングが必要",
     "modelMappingOffHint": "プロバイダーが Anthropic Messages で claude-* / anthropic/claude-* のモデル ID を公開し、そのまま受け付ける場合に使います。Claude Desktop はプロバイダーへ直接接続します。",
     "modelMappingOnHint": "Claude Desktop には claude-* のルート名だけを見せ、CC Switch のローカルルーティングが実際の上流モデルへマッピングします。ローカルルーティングを起動したままにしてください。",

src/i18n/locales/zh.json

@@ -177,8 +177,6 @@
     "mode": "模型处理方式",
     "modeDirect": "直连",
     "modeProxy": "模型映射",
-    "gatewayBaseUrl": "Gateway Base URL",
-    "bearerToken": "Bearer Token",
     "modelMappingToggle": "需要模型映射",
     "modelMappingOffHint": "适合供应商已经暴露并接受 claude-* / anthropic/claude-* 模型名的 Anthropic Messages API；请求会由 Claude Desktop 直连供应商。",
     "modelMappingOnHint": "Claude Desktop 只看到 claude-* 路由名，CC Switch 本地路由会映射到真实上游模型；需要保持本地路由运行。",