feat(proxy): add full URL mode and refactor endpoint rewriting

- Add `isFullUrl` provider meta to treat base_url as complete API endpoint
- Remove hardcoded `?beta=true` from Claude adapter, pass through from client
- Refactor forwarder endpoint rewriting with proper query string handling
- Block provider switching when proxy is required but not running
- Add full URL toggle UI in endpoint field with i18n (zh/en/ja)

src-tauri/src/provider.rs

@@ -245,6 +245,9 @@ pub struct ProviderMeta {
     /// Claude 认证字段名（"ANTHROPIC_AUTH_TOKEN" 或 "ANTHROPIC_API_KEY"）
     #[serde(rename = "apiKeyField", skip_serializing_if = "Option::is_none")]
     pub api_key_field: Option<String>,
+    /// 是否将 base_url 视为完整 API 端点（不拼接 endpoint 路径）
+    #[serde(rename = "isFullUrl", skip_serializing_if = "Option::is_none")]
+    pub is_full_url: Option<bool>,
     /// Prompt cache key for OpenAI-compatible endpoints.
     /// When set, injected into converted requests to improve cache hit rate.
     /// If not set, provider ID is used automatically during format conversion.