Fix menu misalignment on first open after login

[H09-0]

Summary

Fix #13780 - the menu appears at the wrong horizontal position on the
first click after login, then snaps to the correct position ~1 second
later.

Root cause

Two interacting defects in js/ui/popupMenu.js open():

1. Animation path: _allocationChanged blocked during animation

The _allocationChanged handler is guarded by !this.animating (line 2128).
When animation is enabled and open() runs:

1. this.animating = true is set (line 1851)
2. _calculatePosition() runs but the menu layout is incomplete - the
   applet's _onOpenStateChanged only shows INITIAL_BUTTON_LOAD buttons
   synchronously and defers the rest via Mainloop.idle_add (applet.js:1554)
3. When deferred buttons appear, the allocation changes, but
   _allocationChanged bails because animating is still true
4. Only after animation completes and animating = false does the next
   allocation change trigger _allocationChanged - the ~1 second snap

2. Non-animation path: separate x/y assignment races

With this.actor.x = xPos; this.actor.y = yPos;, setting x fires
notify::allocation -> _allocationChanged -> set_position(x', y'),
then this.actor.y = yPos overwrites the corrected Y with the stale value.

Fix (3 insertions, 2 deletions in popupMenu.js)

Change 1 - Animation path onComplete (line 1859-1862)

onComplete: () => {
    this.animating = false;
    let [xPos, yPos] = this._calculatePosition();
    this.actor.set_position(xPos, yPos);
}

Recalculates and sets the correct position the instant the animation
finishes, no longer relying on a delayed _allocationChanged.

Change 2 - Non-animation path (line 1893-1896)

this.actor.set_position(xPos, yPos);

Atomic set eliminates the race between separate x/y assignments and the
synchronous _allocationChanged they trigger.

Testing

1. Centre menu button on panel, reboot, click menu - correct on first click
2. No visual diff on subsequent opens
3. Test with animations enabled and disabled
4. Test with side-panel layouts

Side effects

None. Adds a position recalibration at the natural completion point of the
animation (idempotent). Replaces racy two-step assignment with an atomic
call already used in this class (shiftToPosition, _allocationChanged).