## Summary
This PR fixes the issue with main Zed window which cannot be moved by
dragging the title bar on Windows.

## Root cause 

`is_movable: false` was set for macOS but breaks Windows
`crates/zed/src/zed.rs:381` sets `is_movable: false` for the main
window, introduced by commit `ec7c11c65c` (PR #59836) to fix macOS 27
Beta titlebar click delays. The comment says "disable AppKit's titlebar
dragging", but the setting applies to all platforms, including Windows.

## Why it breaks on Windows

On Windows, window dragging relies on `WM_NCHITTEST` returning
`HTCAPTION`, not `start_window_move()` (which is an empty no-op on
Windows - `crates/gpui/src/platform.rs:703`, never overridden in
`gpui_windows`).
The hit test at `crates/gpui_windows/src/events.rs:903-904` gates
`HTCAPTION` on `is_movable`:

```rust
WindowControlArea::Drag if self.is_movable => Some(HTCAPTION as _),
WindowControlArea::Drag => None,
```

Since `is_movable` is false, `HTCAPTION` is never returned, the title
bar is treated as client area (`HTCLIENT`), mouse events go to gpui's
`on_mouse_move` handler
(`crates/platform_title_bar/src/platform_title_bar.rs:218`), which calls
`window.start_window_move()`, which does nothing on Windows.

On macOS this works because `start_window_move()` IS implemented
(`crates/gpui_macos/src/window.rs:1801`, using
`performWindowDragWithEvent`).

## Test
The fix was successfully built and tested on Windows 11 platform.

---

Release Notes:

- N/A