Bring back preview links as a Share dialog in the header

[shaunandrews]

Related issues

• STU-2162
• Stacked on Collapse push and pull into one Sync action with the designed dialog #4447 (→ Replace the site dropdown with a permanent header #4445 → Agentic UI: Port selective sync #4377). Retarget up the chain as each lands.

⚠️ Visual change: needs human review in light + dark mode.

How AI was used in this PR

Claude Code built this on top of the header stack and iterated on it live in the running app; verified with typecheck and unit tests (the Share dialog's own test passes as-is).

Proposed Changes

Preview links — dropped from the header in the first PR of this stack — come back as a Share button in the header, opening a dedicated dialog. Sharing a site isn't a sync (it publishes a throwaway copy on WordPress.com), so it sits beside the primary action rather than inside the sync flow.

The dialog has two divided sections:

• Live — every WordPress.com site this Studio site is connected to, each with a copy button.
• Preview links — the temporary hosted copies, each with copy, open, and delete in an overflow menu, plus the account's preview-link quota and a New preview action.

Rows are flat and divided by a hairline rather than boxed, so the list reads as a list. The Sync dialog is also pinned to the same 480px width as this one, so the two header dialogs match.

Delete needed a small addition: a deletePreviewSite(hostname) connector method (wired to the existing snapshot-delete IPC / local route) and a useDeletePreviewSite mutation, plus an aboveOverlays option on the menu so an overflow opened inside the dialog isn't trapped behind its scrim.

Testing Instructions

1. Open a connected site → click Share in the header.
2. Confirm the Live section lists the connected WordPress.com sites with working copy buttons.
3. Confirm Preview links lists existing previews; try copy, open, and delete (⋮) on one, and confirm the quota and New preview work.
4. Open the Sync dialog and confirm it's the same width as Share.
5. Both light and dark.

Pre-merge Checklist

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.
• I have added or updated tests to cover my changes, if applicable.
• I have tested my changes in both light and dark mode.