ui(settings): theme picker exit motion + trigger expand parity

[ZhuchkaTriplesix]

Parent

Part of epic #490.

Problem

1. Theme picker closes with _controller.close() immediately — no exit fade-slide. QueryaDropdown delays close so overlay can animate.
2. Trigger label is always Expanded, unlike QueryaDropdown._triggerLabelText (expand only when width / expandToParent). Forces full-width when compact.

Proposal

• Reuse dropdown exit pattern (closeOnActivate: false + delayed close via motionDuration(standard))
• Expand trigger label only when expandToParent || fieldWidth != null

Files

• lib/features/settings/theme_picker_button.dart
• lib/shared/widgets/querya_dropdown.dart (reference / shared helper)

Acceptance

• Closing theme menu plays exit fade-slide under Full/Reduced; Off snaps
• Compact trigger does not force Expanded stretch

[ZhuchkaTriplesix]

Shipped on dev via #512.