* New window inherits size from current window
When creating a new window via Cmd+Shift+N, use the key window's
frame dimensions instead of the hardcoded 460x360 default. The new
window cascades from the existing window's position so it doesn't
stack directly on top.
* Use Ghostty's cascade algorithm for new window positioning
Match upstream Ghostty's window cascade logic: maintain a
lastCascadePoint that tracks where the next window should appear.
First window seeds the point from its own top-left corner, subsequent
windows advance the cascade point via NSWindow.cascadeTopLeft(from:).
On window close, reset the cascade point to the closing window's
position so the next window appears nearby.
New windows still inherit the key window's size so Cmd+Shift+N
creates a window matching the previous one's dimensions.
* Fix frame-to-contentRect conversion and use preferred window resolver
Convert existingFrame to a content rect via
NSWindow.contentRect(forFrameRect:styleMask:) so the new window
matches the source window's actual size instead of growing by
titlebar insets on each Cmd+Shift+N.
Use preferredMainWindowContextForWorkspaceCreation to resolve the
source window, consistent with showOpenFolderPanel and other
callers.
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Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>
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