fix: notification items are now real links - #2039

Summary

• Notification items in the notification panel were rendered as <button> elements with a useNavigate() click handler, making it impossible to open them in a new tab or window.
• Replaced UnstyledButton (button) with UnstyledButton component={Link} so each item renders as a real <a> element — the browser handles all navigation natively.
• Regular left-click still uses SPA navigation and closes the popover.
• Ctrl/Cmd/Shift+click and middle-click open the page in a new tab; all of these mark the notification as read.
• Right-click (context menu) does not mark as read.
• Added display: block to the CSS class to fix layout regression caused by <a> defaulting to display: inline (unlike <button>).

Notes

⚠️ This code was written by an AI assistant (Claude) and has been manually tested by a human. Please review carefully before merging.

Test plan

• Left-click a notification — navigates in-place, marks as read, closes popover
• Ctrl/Cmd+click — opens in new tab, marks as read
• Middle-click — opens in new tab, marks as read
• Right-click — opens context menu, does not mark as read
• Notification items display correctly (no extra spacing, hover highlight is a single block)

🤖 Generated with Claude Code