Part of #29026

## Summary

https://github.com/user-attachments/assets/35b4f969-1fcf-45f4-88cd-cbc27ad9696e

macOS Finder places file paths on the system pasteboard using
`NSFilenamesPboardType` when files are copied. Previously, the project
panel only supported its own internal clipboard for copy/cut/paste
operations and ignored system clipboard content entirely. This meant
that copying files in Finder and pasting them in Zed's project panel did
nothing.

This PR adds support for reading file paths from the macOS system
pasteboard, enabling a natural workflow where users can copy files in
Finder (or other file managers) and paste them directly into Zed's
project panel.

> **Note:** Pasting files from a system file manager currently only
works on macOS. The project panel changes are cross-platform, but the
clipboard reading of file paths (`ExternalPaths`) is only implemented in
the macOS pasteboard. Windows and Linux would need equivalent changes in
their respective platform clipboard implementations to support this.
Copying/cutting files from the project panel to the system clipboard as
plain text works on all platforms.

### Changes

**`crates/gpui/src/platform/mac/pasteboard.rs`**
- Read `NSFilenamesPboardType` from the system pasteboard and surface
file paths as `ClipboardEntry::ExternalPaths`
- Check for file paths before plain text, since Finder puts both types
on the pasteboard (without this priority, file paths would be returned
as plain text strings)
- Extract the string-reading logic into `read_string_from_pasteboard()`
to allow reuse

**`crates/project_panel/src/project_panel.rs`**
- On paste, check the system clipboard for external file paths and use
the existing `drop_external_files` mechanism to copy them into the
project
- On copy/cut, write the selected entries' absolute paths to the system
clipboard so other apps can consume them
- Update the "Paste" context menu item to also be enabled when the
system clipboard contains file paths, not just when the internal
clipboard has entries

## Test plan

- [ ] Copy one or more files in Finder, paste in the project panel —
files should be copied into the selected directory
- [ ] Copy files within the project panel, paste — existing internal
copy/paste behavior is preserved
- [ ] Cut files within the project panel, paste — existing internal
cut/paste behavior is preserved
- [ ] Copy files in the project panel, paste in Finder or another app —
paths are available as plain text
- [ ] Right-click context menu shows "Paste" enabled when system
clipboard has file paths
- [ ] Right-click context menu shows "Paste" disabled when both internal
and system clipboards are empty

Release Notes:

- Added support for pasting files from Finder (and other file managers)
into the project panel via the system clipboard (macOS only). Copying or
cutting files in the project panel now also writes their paths to the
system clipboard for use in other apps.

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Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>