quickinput: keep Command Center visible when Quick Pick is dragged away (#306139)

* quickinput: expose alignment observable, keep Command Center visible when Quick Pick is dragged

When the Quick Pick is dragged away from its default top position, the
Command Center in the title bar no longer hides. Previously it always
hid on Quick Pick show, even when the widget was in a custom position
where it wouldn't overlap.

Changes:
- Add QuickInputAlignment type ('top' | 'center' | 'custom') and
  alignment observable to IQuickInputService
- DnD controller tracks alignment via _setAlignmentState() helper that
  updates both the context key and the observable
- Each service layer has a stable observableValue mirrored via autorun
  (avoids breaking subscriptions on lazy controller creation)
- Command Center uses autorun to reactively show/hide based on alignment
- Fix onShowEmitter.fire() timing (moved after layoutContainer so
  alignment is settled before listeners fire)
- Fix top===0 truthiness bugs in layoutContainer and updateLayout
- Fix double-click reset not updating alignment state

Fixes #306138

Co-authored-by: Copilot <[email protected]>

* quickinput: handle anchored inputs and partial state in alignment

- Set alignment to 'custom' for anchored quick inputs (positioned near
  a DOM element, not at the top)
- Re-sync alignment from DnD controller on non-anchored show to prevent
  stale 'custom' value after an anchored input closes
- Guard setAlignment() to no-op while an anchored input is visible
- DnD alignment typed as IObservable<QuickInputAlignment> (read-only)
- Require both top and left in persisted state before marking as custom

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: Copilot <[email protected]>