feat: add auto-fetch models from provider's /v1/models endpoint

Add ability to fetch available models from third-party aggregation
providers (SiliconFlow, OpenRouter, etc.) via OpenAI-compatible
GET /v1/models endpoint. Users can click "Fetch Models" button in
the provider form, then select models from a dropdown on each
model input field.

- Backend: new model_fetch service + Tauri command (Rust)
- Frontend: ModelInputWithFetch shared component
- Integrated into all 5 app forms (Claude/Codex/Gemini/OpenCode/OpenClaw)
- i18n support for zh/en/ja

src/lib/api/model-fetch.ts

@@ -0,0 +1,20 @@
+import { invoke } from "@tauri-apps/api/core";
+
+export interface FetchedModel {
+  id: string;
+  ownedBy: string | null;
+}
+
+/**
+ * 从供应商获取可用模型列表
+ *
+ * 使用 OpenAI 兼容的 GET /v1/models 端点。
+ * 主要面向第三方聚合站（硅基流动、OpenRouter 等）。
+ */
+export async function fetchModelsForConfig(
+  baseUrl: string,
+  apiKey: string,
+  isFullUrl?: boolean,
+): Promise<FetchedModel[]> {
+  return invoke("fetch_models_for_config", { baseUrl, apiKey, isFullUrl });
+}