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/types.ts

@@ -269,6 +269,8 @@ export interface OpenCodeProviderOptions {
   baseURL?: string;
   apiKey?: string;
   headers?: Record<string, string>;
+  // 支持额外选项（timeout, setCacheKey 等）
+  [key: string]: unknown;
 }
 
 // OpenCode 供应商配置（settings_config 结构）