refactor(codex): extract template to config with i18n support

**Changes:**
- Create src/config/codexTemplates.ts with getCodexCustomTemplate factory
- Support both Chinese and English templates based on i18n.language
- Remove 70 lines of duplicated template strings from ProviderForm.tsx
- Update both useEffect and handlePresetChange to use template factory
- Clean up unused "Custom (Blank Template)" preset entry

**Benefits:**
- ✅ Eliminates code duplication (35-line template repeated twice)
- ✅ Adds internationalization support for English users
- ✅ Follows project architecture (templates in config/ directory)
- ✅ Improves maintainability (single source of truth)
- ✅ Net reduction: 34 lines (81 additions, 115 deletions)

**Technical Details:**
- Template selection logic: (i18n.language || "zh").startsWith("zh") ? "zh" : "en"
- Templates are identical except for comments language
- Both auth and config are returned as a single CodexTemplate object

Addresses DRY principle violation and architectural concerns identified
in code review.

src/config/codexProviderPresets.ts

@@ -98,52 +98,6 @@ requires_openai_auth = true`,
       textColor: "#FFFFFF",
     },
   },
-  {
-    name: "Custom (Blank Template)",
-    websiteUrl: "https://docs.anthropic.com",
-    category: "third_party",
-    isCustomTemplate: true,
-    auth: generateThirdPartyAuth(""),
-    config: `# ========================================
-# Codex 自定义供应商配置模板
-# ========================================
-# 快速上手：
-# 1. 在上方 auth.json 中设置 API Key
-# 2. 将下方 'custom' 替换为供应商名称（小写、无空格）
-# 3. 替换 base_url 为实际的 API 端点
-# 4. 根据需要调整模型名称
-#
-# 文档: https://docs.anthropic.com
-# ========================================
-
-# ========== 模型配置 ==========
-model_provider = "custom"        # 供应商唯一标识
-model = "gpt-5-codex"            # 模型名称
-model_reasoning_effort = "high"  # 推理强度：low, medium, high
-disable_response_storage = true  # 隐私：不本地存储响应
-
-# ========== 供应商设置 ==========
-[model_providers.custom]
-name = "custom"                                    # 与上方 model_provider 保持一致
-base_url = "https://api.example.com/v1"           # 👈 替换为实际端点
-wire_api = "responses"                            # API 响应格式
-requires_openai_auth = true                       # 使用 auth.json 中的 OPENAI_API_KEY
-
-# ========== 可选：自定义请求头 ==========
-# 如果供应商需要自定义请求头，取消注释：
-# [model_providers.custom.headers]
-# X-Custom-Header = "value"
-
-# ========== 可选：模型覆盖 ==========
-# 如果需要覆盖特定模型，取消注释：
-# [model_overrides]
-# "gpt-5-codex" = { model_provider = "custom", model = "your-model-name" }`,
-    theme: {
-      icon: "generic",
-      backgroundColor: "#6B7280", // gray-500
-      textColor: "#FFFFFF",
-    },
-  },
   {
     name: "AiHubMix",
     websiteUrl: "https://aihubmix.com",