fix(studio): larger default window — 1440x900 across all platforms

Stefan observed the default Studio window opened smaller
than comfortable when launched fresh (no persisted size).
On macOS specifically the xib-stored default was around
800x600 — too tight to read the sidebar + content + run
panel side-by-side.

Set 1440x900 as the initial content size across all three
host platforms:

 - macOS: MainFlutterWindow.swift sets contentSize via
   NSWindow.setContentSize after the FlutterViewController
   attaches, and centers on the active NSScreen. Clamped to
   the screen's visibleFrame so we never open larger than
   the display (matters for non-Retina external monitors).
 - Linux: gtk_window_set_default_size 1280x720 -> 1440x900
 - Windows: Win32Window::Size 1280x720 -> 1440x900

1440x900 is the effective Retina resolution of a 13" MacBook
(Stefan's dev machine) and the smallest "modern desktop"
footprint that lets the sidebar + content + tool panel
breathe.

Version 0.51.8 -> 0.51.9.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

windows/runner/main.cpp

@@ -26,7 +26,7 @@ int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
 
   FlutterWindow window(project);
   Win32Window::Point origin(10, 10);
-  Win32Window::Size size(1280, 720);
+  Win32Window::Size size(1440, 900);
   if (!window.Create(L"F\u2206I Studio", origin, size)) {
     return EXIT_FAILURE;
   }