feat(refactor): extract a reusable `Spinner` component to replace duplicated spinner markup across the codebase.

## Summary
Extract a reusable `Spinner` component to replace duplicated spinner markup across the codebase.

## Background

During review of PR #214, it was noted that spinner markup is duplicated in multiple places with slight variations. This creates maintenance overhead and inconsistency.

## Current State

Spinner markup is duplicated in:
- `ResumeCard.tsx` - Preview loading overlay (h-8 w-8)
- `ResumeCard.tsx` - Edit button loading (h-4 w-4)
- `MobileActionBar.tsx` - Preview button (h-6 w-6)
- `SectionNavigator.tsx` - Preview button (h-4 w-4)

Each uses slightly different sizes but the same pattern:
```tsx

Proposed Solution

Create a reusable Spinner component:

// components/Spinner.tsx
interface SpinnerProps {
size?: 'sm' | 'md' | 'lg';
color?: 'white' | 'blue';
className?: string;
}

export function Spinner({ size = 'md', color = 'white', className }: SpinnerProps) {
const sizeClasses = {
sm: 'h-4 w-4',
md: 'h-6 w-6',
lg: 'h-8 w-8'
};

const colorClasses = {
white: 'border-white border-t-transparent',
blue: 'border-blue-600 border-t-transparent'
};

return (