[add] Spinner & Loading components

[Copilot]

Consolidates scattered animate-spin implementations into a reusable Spinner component and adds a Loading overlay component based on the Idea-React reference.

Changes

• New Spinner component - Centralizes all spinner animations with configurable sizes (sm/md/lg) and accessibility attributes
• New Loading component - Full-screen overlay using Spinner for blocking operations
• Refactored components - Replaced inline animate-spin divs in searchable-input, rest-table, and lucide-react's Loader2 in image-preview with Spinner
• Registry updates - Added both components to shadcn registry with proper dependency tracking

Usage

// Simple spinner
<Spinner size="sm" />
<Spinner className="text-primary" />

// Full-screen loading overlay
<Loading>Processing data...</Loading>

All existing functionality preserved - changes are drop-in replacements.

Original prompt

当整个项目中的 animate-spin 抽出一个通用的 <Spinner /> 组件，并基于此实现：https://github.com/idea2app/Idea-React/blob/master/source%2FLoading.tsx

Human changes

1. [optimize] simplify Copilot's source codes

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