🎨 Palette: 为异步提交按钮添加平滑过渡效果

[01luyicheng]

💡 What: 使用 AnimatedContent 和 Row 包装配对按钮内容，并将加载指示器的颜色改为 LocalContentColor.current。
🎯 Why: 避免状态切换时的突兀视觉变化，并修复禁用状态下对比度不足的问题。
📸 Before/After: （纯视觉动画和对比度改进）
♿ Accessibility: 提高了禁用状态下 Loading 图标的视觉对比度，确保键盘焦点状态更清晰。

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PR created automatically by Jules for task 17176571655609954033 started by @01luyicheng

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[Luyicheng-Agent]

🤖 自动提交后正确性审查（过去 24 小时提交） — commit 44f379c (2026-08-06)

对本 PR 进行了多维度追踪审查，结论：未发现高影响缺陷（无崩溃 / 数据丢失 / 安全 / 严重功能退化）。

审查要点：

1. AnimatedContent 默认 transitionSpec（crossfade+scale）——与文件中已有的 ConnectionStatusAnimated/VpnStatusCard 模式一致（见 REV62 注释），过渡期间两状态叠加淡入淡出，无空白闪烁。
2. LocalContentColor.current 语义——spinner 仅在 isPairingInProgress=true 时显示，此时 enabled = … && !isPairingInProgress = false（按钮 disabled）。Material3 Button 在 disabled 态提供 LocalContentColor = onSurface@0.38，容器为 onSurface@0.12，对比度良好且可见。旧代码用 onPrimary（白色）在浅灰 disabled 容器上反而对比度不足——本 PR 实际修复了该问题。
3. Row 包裹——AnimatedContent 移除了隐式 RowScope，显式 Row(Center) 正确保留了 spinner+text 并排居中，无窄屏裁剪。
4. 状态重组——Boolean targetState 以 == 触发过渡，lambda 内使用局部 isPairing 参数（非捕获的 uiState），模式正确。

唯一建议（非缺陷）：LocalContentColor 依赖 import androidx.compose.material3.* 通配符解析（第 43 行）。当前可正确编译（material3 包导出该 API），但建议显式 import androidx.compose.material3.LocalContentColor 以提升可读性、避免未来包重构时的隐式依赖。

另：未发现 PairingSection 相关 Compose UI 测试覆盖，建议后续补充（非阻塞）。CI 当前 pending，待 Android Build & Test 通过后可最终确认编译。