Re-subscribe theme updates on attach for symmetric lifecycle

[VPDPersonal]

Summary

• Make AddAspidThemeStyleSheets subscribe to AspidThemeSettings.Changed on AttachToPanelEvent (re-applying the current override there) instead of once at build time, and unsubscribe on DetachFromPanelEvent.
• Fixes live theme updates breaking after any detach -> reattach (Inspector foldout collapse/expand, ListView row recycling) and the static-event leak for elements that are built but never attached.

Notes for review

• Re-applying the override inside OnThemeChanged on attach is idempotent: it removes the previously applied sheet (if any) before re-reading and re-adding the current override, so repeated attach/detach cycles do not stack duplicate style sheets.
• Mirrors the attach/detach-symmetric pattern already used by IdStructUIToolkitPropertyDrawer.

Linked issues

Refs #49 - addresses review finding #49 (comment)