[FEATURE] Add toast notification after copy action

[ascender1729]

Description

Show a brief toast/snackbar notification when content is copied to clipboard.

Why Needed

• Confirms action was successful
• Standard UX feedback pattern
• Prevents user uncertainty

Implementation Details

• Create reusable Toast component
• Show 'Copied!' message for 2-3 seconds
• Position at bottom or top of screen
• Auto-dismiss with fade animation

Files to Create/Modify

• src/components/shared/Toast.svelte (new)
• src/lib/toast.ts (toast store)
• Update copy button components

Code Example

<script>
  import { showToast } from '$lib/toast';
  
  function handleCopy() {
    navigator.clipboard.writeText(url);
    showToast('Copied to clipboard!');
  }
</script>

Acceptance Criteria

• Toast appears on copy action
• Auto-dismisses after delay
• Smooth animation in/out
• Accessible (aria-live region)

Time Estimates

• With AI: 100 minutes
• Without AI: 660 minutes