fix: Window menu tiling items are disabled

[exequiellares]

What this does

The Window menu's Fill, Center, Move & Resize, Full Screen Tile and Move to Display items (and their shortcuts) are greyed out in Programa. This makes them work again.

Summary

macOS enables those items only when the window reports isMovable == true. The main window was created with isMovable = false (from the cmux-era titlebar drag fix, and re-asserted in the chrome configure pass), which disables the whole tiling family. The flag was off to stop clicks under the transparent titlebar from turning into native window drags; that is now handled by MainWindowHostingView returning false from mouseDownCanMoveWindow, the same approach upstream Ghostty uses with its NonDraggableHostingView. isMovableByWindowBackground stays off, and the explicit performDrag paths are unchanged.

Test plan

• Open the Window menu on a main window: Fill, Center, Move & Resize and Move to Display are enabled and act on the window.
• ⌃⌥⌘F and the Move & Resize shortcuts tile the window.
• Minimal sidebar mode: clicking sidebar buttons under the titlebar activates them and does not drag the window.
• Dragging empty titlebar space still moves the window; dragging a sidebar tab reorders it without moving the window.
• Dragging the titlebar folder icon still starts a file drag, not a window move.