feat(ux): 骨架屏加载组件与页面入场动画

新增 Skeleton 组件支持 text/card/table 三种变体，替换各页面
"加载中..." 文本为骨架屏，所有页面添加 fadeIn 入场动画。

frontend/src/pages/GeneratePage.tsx

@@ -45,7 +45,7 @@ export default function GeneratePage() {
   }
 
   return (
-    <div className="container" style={{ paddingTop: 24, paddingBottom: 40 }}>
+    <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
       <h1 style={{ fontSize: 24, marginBottom: 32 }}>新建生成</h1>
 
       {/* 生成表单 */}

frontend/src/pages/LoginPage.tsx

@@ -27,7 +27,7 @@ export default function LoginPage() {
         justifyContent: 'center',
       }}
     >
-      <div className="card" style={{ width: 380, padding: 40 }}>
+      <div className="card page-enter" style={{ width: 380, padding: 40 }}>
         <h1 style={{ textAlign: 'center', marginBottom: 8, fontSize: 28 }}>gen2d</h1>
         <p
           style={{

frontend/src/pages/ProjectPage.tsx

@@ -4,6 +4,7 @@ import { useProjectStore } from '../stores/project'
 import { getTasks } from '../api/project'
 import type { Task } from '../api/types'
 import StyleSelector from '../components/StyleSelector'
+import Skeleton from '../components/Skeleton'
 
 const STATUS_LABELS: Record<string, { label: string; color: string }> = {
   pending: { label: '等待中', color: 'var(--text-muted)' },
@@ -42,8 +43,8 @@ export default function ProjectPage() {
   }
 
   return (
-    <div className="container" style={{ paddingTop: 24, paddingBottom: 40 }}>
-      <h1 style={{ fontSize: 24, marginBottom: 32 }}>{loading ? '加载中...' : name}</h1>
+    <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
+      <h1 style={{ fontSize: 24, marginBottom: 32 }}>{loading ? <Skeleton variant="text" lines={1} /> : name}</h1>
 
       {/* 工程风格 */}
       <section className="card" style={{ marginBottom: 24 }}>

frontend/src/pages/RegisterPage.tsx

@@ -28,7 +28,7 @@ export default function RegisterPage() {
         justifyContent: 'center',
       }}
     >
-      <div className="card" style={{ width: 380, padding: 40 }}>
+      <div className="card page-enter" style={{ width: 380, padding: 40 }}>
         <h1 style={{ textAlign: 'center', marginBottom: 8, fontSize: 28 }}>gen2d</h1>
         <p
           style={{

frontend/src/pages/ResultPage.tsx

@@ -3,6 +3,7 @@ import { Link, useParams } from 'react-router-dom'
 import { getTask, getAssets } from '../api/generate'
 import type { Asset, Task } from '../api/types'
 import AssetPreview from '../components/AssetPreview'
+import Skeleton from '../components/Skeleton'
 
 export default function ResultPage() {
   const { projectId = 'proj-default', taskId } = useParams()
@@ -23,8 +24,16 @@ export default function ResultPage() {
 
   if (loading) {
     return (
-      <div className="container" style={{ paddingTop: 40, textAlign: 'center' }}>
-        <p style={{ color: 'var(--text-secondary)' }}>加载中...</p>
+      <div className="container page-enter" style={{ paddingTop: 40 }}>
+        <div className="card" style={{ marginBottom: 24 }}>
+          <Skeleton variant="text" lines={1} />
+          <div style={{ marginTop: 16 }}>
+            <Skeleton variant="text" lines={4} />
+          </div>
+        </div>
+        <div className="card">
+          <Skeleton variant="card" />
+        </div>
       </div>
     )
   }
@@ -41,7 +50,7 @@ export default function ResultPage() {
   }
 
   return (
-    <div className="container" style={{ paddingTop: 24, paddingBottom: 40 }}>
+    <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
       <h1 style={{ fontSize: 24, marginBottom: 32 }}>生成结果</h1>
 
       {/* 任务信息 */}