## Context

`ctrl-n` / `cmd-n` doesn't work on the Welcome tab. The global binding
lives under `Workspace && !Terminal` (macOS) and similar contexts that
don't include Welcome. The fix just adds the same binding to the Welcome
context block on all three platforms — same approach the font-size and
recent-project shortcuts already use there.

Closes #52426

## Demo

### Before:




https://github.com/user-attachments/assets/69becde8-25d2-45e3-9e7c-416b7937bd17

### After:




https://github.com/user-attachments/assets/6d9ede76-7adb-4527-bfef-c18d5b8a4fb4









## How to review

One line added per platform keymap file. Check that `ctrl-n` / `cmd-n`
maps to `workspace::NewFile` in the `Welcome` block of:
- `assets/keymaps/default-macos.json`
- `assets/keymaps/default-linux.json`
- `assets/keymaps/default-windows.json`

## Self-review checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed `ctrl-n` / `cmd-n` (New File) not working on the Welcome tab

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>