Add cross-window file/folder copy-paste support via system clipboard

[mxvsh]

Summary

This PR adds support for copying and pasting files/folders across different Zed windows using the system clipboard.

Previously, the paste button was disabled when trying to paste files copied from another Zed window. This implementation enables:

• Cross-window clipboard: Copy files in one Zed window and paste them in another
• Cross-instance support: Works between different Zed instances
• External file integration: Can paste file paths copied from system file managers (Finder/Explorer)
• Cut/Copy distinction: Preserves whether files were cut or copied

Implementation

• Modified copy() and cut() to write file paths to system clipboard with format: ZED_CLIPBOARD:CUT|COPY\n/path1\n/path2...
• Added has_clipboard_content() to check both local and system clipboard
• Added parse_system_clipboard() to parse Zed format or external file paths
• Updated paste() to check local clipboard first, then fall back to system clipboard
• Added write_entries_to_system_clipboard() helper function

Test Plan

1. Open two Zed windows
2. Copy files/folders in window A (Cmd+C / Ctrl+C)
3. Switch to window B
4. Right-click in project panel → Paste option should now be enabled
5. Paste successfully copies the files to the target location

Release Notes:

• Added cross-window file/folder copy-paste support in project panel via system
  clipboard

🤖 Generated with Claude Code

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @mxvsh on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @mxvsh on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[mxvsh]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[spandan-kumar]

looks clean