feat(opencode): add extra options editor for SDK configuration

Add key-value pair editor for configuring additional SDK options like
timeout, setCacheKey, etc. Values are automatically parsed to appropriate
types (number, boolean, object) on save.

- Add `extra` field with serde flatten in Rust backend
- Add index signature to OpenCodeProviderOptions type
- Create ExtraOptionKeyInput component with local state pattern
- Place extra options section above models configuration

src-tauri/src/provider.rs

@@ -521,6 +521,11 @@ pub struct OpenCodeProviderOptions {
     /// 自定义请求头
     #[serde(skip_serializing_if = "Option::is_none")]
     pub headers: Option<HashMap<String, String>>,
+
+    /// 额外选项（timeout, setCacheKey 等）
+    /// 使用 flatten 捕获所有未明确定义的字段
+    #[serde(flatten, default, skip_serializing_if = "HashMap::is_empty")]
+    pub extra: HashMap<String, Value>,
 }
 
 /// OpenCode 模型定义