fix: Claude Desktop 官方供应商添加报错 #3402 (#3405)

* fix: Claude Desktop 官方供应商添加时缺少 ANTHROPIC_BASE_URL 报错

根因：前端 mutation 为 claude-desktop 生成随机 UUID 作为 provider id，
后端 is_official_provider 通过 id 匹配跳过校验，随机 UUID 不匹配导致
走入普通 direct 模式校验并要求 ANTHROPIC_BASE_URL。

修复：
- 前端：claude-desktop + category=official 时使用固定 id "claude-desktop-official"
- 后端：validate_provider / validate_direct_provider / validate_proxy_provider /
  apply_provider_to_paths 增加 category=="official" 兜底检查

Fixes #3402

* fix: restrict Claude Desktop official provider detection

* fix: add Claude Desktop official provider via seed

---------

Co-authored-by: 金恩光 <enguang.jin@gmail.com>
Co-authored-by: Jason <farion1231@gmail.com>

src-tauri/src/commands/provider.rs

@@ -219,6 +219,17 @@ pub fn import_claude_desktop_providers_from_claude(
     Ok(imported)
 }
 
+#[tauri::command]
+pub fn ensure_claude_desktop_official_provider(state: State<'_, AppState>) -> Result<bool, String> {
+    state
+        .db
+        .ensure_official_seed_by_id(
+            crate::database::CLAUDE_DESKTOP_OFFICIAL_PROVIDER_ID,
+            AppType::ClaudeDesktop,
+        )
+        .map_err(|e| e.to_string())
+}
+
 fn claude_provider_models_are_claude_safe(provider: &Provider) -> bool {
     let Some(env) = provider
         .settings_config