Dashboard: add copy share link and open buttons to modpack rows

[kytxii]

The dashboard modpack table is currently read-only — rows show name, version, loader, and share code but offer no way to act without navigating away.

Requirements

• Add a "Copy" icon button next to the share code in each row — copies the full share URL (/share/<code>) to the clipboard and shows "Copied!" for 2 seconds
• Add an "Open" button at the end of each row that navigates to /modpacks/<id>
• Keep the row itself non-navigating — clicking a row should not navigate anywhere

Implementation notes

• Changes in frontend/src/pages/Dashboard.jsx
• Share URL: window.location.origin + "/share/" + share_code
• Use navigator.clipboard.writeText; fall back to document.execCommand("copy") or silently fail in non-HTTPS environments
• Hide the copy button when share_code is null

Definition of done

• Each row has a copy-share-code button with "Copied!" feedback
• Each row has an "Open" button navigating to the modpack builder
• Clicking the row itself does not navigate