Window: remember the window's size and position between launches

The window opened at a fixed 1680 × 1020 wherever the system put it. Its size, position
and maximized state are now saved to window.json in the config directory (when they change,
and on exit) and restored on the next launch, moved onto a monitor that is still connected.
The first launch fits the window to its screen. Agent-driven runs (--control) restore the
saved window but don't overwrite it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.lock

@@ -1159,6 +1159,7 @@ dependencies = [
  "objc2-foundation 0.3.2",
  "pollster 0.4.0",
  "rfd",
+ "serde",
  "serde_json",
  "winit",
  "winresource",