…#2367)

On macOS, AppKit delivers Qt::WindowStateChange asynchronously, so the
Maximized/FullScreen bit could survive setFixedWidth(260) inside
toggleMinimalMode(true). The deferred WindowStateChange then arrived
with m_minimalMode==true and the Maximized bit still set, causing
changeEvent to schedule a spurious toggleMinimalMode(false) — the
window collapsed for a frame and snapped back to the full-size geometry
restored from FullModeGeometry.

- toggleMinimalMode(true): drop maximized/fullscreen state via
  showNormal() before forcing the applet width. FullModeGeometry is
  saved first, so exit can still restore the user's pre-minimal window.
- Add m_enteringMinimalMode guard symmetric to m_exitingMinimalMode and
  honor it in changeEvent. Cleared via singleShot(0) so any AppKit-
  deferred WindowStateChange queued during the enter path drains through
  the early-return before the guard drops.

Both behaviors are no-ops on Windows/Linux where setFixedWidth on a
maximized top-level synchronously clears the state bit.

Regressed in #2299, which collapsed minimal-exit paths into the single
changeEvent branch and exposed the missing entering-side guard.

Co-authored-by: AetherClaude <[email protected]>
Co-authored-by: Claude Opus 4.7 <[email protected]>