refactor(frontend): consolidate features & components into single tree

把 components/admin/* 和 components/platform/* 共 11 个文件
移到对应 features/<name>/ 下,components/ 只保留跨特性共享的
common/ 子目录(features/admin/* 之前和 components/admin/* 同名
共存,迫使 features/admin/AdminPages.tsx 出现 barrel,顺带消除了
barrel 副作用 CSS 丢失的隐患)。

新的目录约定:
  - features/<name>/  = 特性模块,所有页面 / Modal / state / hooks
                         / routes 都在内,无 components/<name>/ 并列
  - components/common/ = 仅放 ≥2 个特性共用的 widgets
                         (Icon、Sidebar、Toast、Topbar、WelcomePanel)

改动:
  - 11 文件 git mv (components/{admin,platform}/* → features/*)
  - 8 文件的 Icon 相对路径 '../common/Icon' → '../../components/common/Icon'
  - 2 文件补 admin.css side-effect import(UserManagementPage /
    ProjectManagementPage 之前依赖 AdminPages barrel)
  - 3 文件(引用方)更新 import:SystemAdminPage、DashboardRoute、
    ScriptsPage
  - 删除 features/admin/AdminPages.tsx barrel(无消费者)
  - 删除空目录 components/admin、components/platform

验证:
  - pnpm typecheck 通过
  - pnpm build 通过,CSS 体积 35.11 kB 与重构前一致(无样式增减)
  - routes.ts / routes/platform.tsx 路径未动,route id 不变

踩坑:
  第一轮把 '../common/Icon' 错改成 '../../../components/common/Icon',
  typecheck 报 11 个 Cannot find module。原因:features/admin/ 和
  components/admin/ 都是 depth 2,'../../X' 在两边都解析到 app/X,
  只有 '../X' 才需要多一层。正确改法是 '../../components/common/Icon'。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/app/features/platform/CreateFolderModal.tsx

@@ -0,0 +1,80 @@
+import { type FormEvent } from "react";
+import Icon from "../../components/common/Icon";
+
+type CreateFolderModalProps = {
+  open: boolean;
+  parentPath: string;
+  name: string;
+  busy: boolean;
+  onNameChange: (name: string) => void;
+  onSubmit: (event: FormEvent) => void;
+  onClose: () => void;
+};
+
+export function CreateFolderModal({
+  open,
+  parentPath,
+  name,
+  busy,
+  onNameChange,
+  onSubmit,
+  onClose,
+}: CreateFolderModalProps) {
+  if (!open) return null;
+
+  return (
+    <div className="modal-backdrop" role="presentation">
+      <section className="modal modal--compact" role="dialog" aria-modal="true">
+        <div className="modal__header">
+          <div>
+            <span className="modal__eyebrow">WORKSPACE</span>
+            <h2>新建文件夹</h2>
+          </div>
+          <button
+            className="icon-button"
+            type="button"
+            aria-label="关闭"
+            onClick={onClose}
+          >
+            <Icon name="close" />
+          </button>
+        </div>
+        <form onSubmit={onSubmit}>
+          <div className="destination-chip">
+            <Icon name="folder" size={16} />
+            创建到：{parentPath || "个人根目录"}
+          </div>
+          <label className="form-field">
+            <span>文件夹名称</span>
+            <input
+              autoFocus
+              maxLength={255}
+              placeholder="例如：模型训练"
+              value={name}
+              onChange={(event) => onNameChange(event.target.value)}
+            />
+          </label>
+          <div className="modal__footer">
+            <button
+              className="secondary-button"
+              type="button"
+              onClick={onClose}
+            >
+              取消
+            </button>
+            <button
+              className="primary-button"
+              type="submit"
+              disabled={busy || !name.trim()}
+            >
+              {busy
+                ? <span className="button-spinner" />
+                : <Icon name="folder" size={16} />}
+              {busy ? "正在创建…" : "创建文件夹"}
+            </button>
+          </div>
+        </form>
+      </section>
+    </div>
+  );
+}