feat(admin): 面板补全 — 插件安装表单+已装列表 / LLM组maxTokens展示编辑 / POST admin/llm 部分编辑合并

- 前端: api.ts(adminPluginInstall带config/uninstall/listInstalledPlugins) + types + plugins/InstalledPlugins.tsx(已装列表+安装表单, Miuix) + llm/LlmProviderForm.tsx(maxTokens输入) ; Plugins.tsx去假安装按钮
- 后端 admin_write_llm_channels.go: llmProviderRequest字段改指针, mergeLlmProvider部分合并(省略保留原值), runtimeconfig/models.go Find
- 新增测试 admin_write_test TestAdminAddLlmProviderPartialEditPreservesFields
- 前端 build + 后端 build/vet/test 全绿; Docs/api-client v0.29
- (面板无法实机登录, 用 react-dom/server 冒烟渲染 LlmModule/PluginsModule)

internal/config/runtimeconfig/models.go

@@ -184,6 +184,21 @@ func GroupFromEnv(env config.LLMConfig) []Group {
 	}}
 }
 
+// Find returns the group with the given id, if present. It is the read half of
+// the admin read-modify-write merge, so a partial edit starts from the stored
+// group instead of zero values.
+func Find(groups []Group, id string) (Group, bool) {
+	if strings.TrimSpace(id) == "" {
+		return Group{}, false
+	}
+	for _, g := range groups {
+		if g.ID == id {
+			return g, true
+		}
+	}
+	return Group{}, false
+}
+
 // Upsert returns a copy of groups with g inserted or replaced in place by ID,
 // preserving order. A blank ID is assigned a generated id so a panel save
 // without an explicit id still produces a stable, addressable group.