"Copy All" button on snippet cards

[byteofhoney]

Goal: Let users copy a snippet's code straight from the card grid, without opening it.
User story: As a user, I want a copy button on each card so that I can grab code fast without clicking into the snippet.

Tasks

• Add a small copy button/icon to each .snippet-card
• Clicking it must not trigger navigation into the snippet detail page (stopPropagation)
• Reuse the existing copyCode() pattern/feedback style already used on the detail page

Acceptance Criteria

• Clicking the copy icon on any card copies that snippet's code to clipboard
• Clicking it does not navigate to the snippet detail page
• Button gives the same "Copied!" visual feedback as the existing copy button