Fix dark theme scrollbar colors customizations pages - #333634

Replaces native scrollbars on customization and plugin card pages with VS Code’s themed overlay scrollbar.

Changes:

• Added DomScrollableElement wrappers and scroll handling.
• Added content/layout rescanning.
• Added scrollbar gutter and wrapper styling.

src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:800

• Pass the containing window to DisposableResizeObserver. Without it, the observer is constructed in mainWindow; when this editor is hosted in an auxiliary window, the new card scrollbar will not be observed in the correct DOM realm. Nearby observers already use this pattern (aiCustomizationListWidget.ts:725-739, aiCustomizationManagementEditor.ts:899-905).

		const cardResizeObserver = this._register(new DOM.DisposableResizeObserver(
			'AICustomizationListWidget.cardScrollable',
			() => this.cardScrollable.scanDomNode(),
		));
		this._register(cardResizeObserver.observe(this.cardScrollableNode));

src/vs/workbench/contrib/chat/browser/aiCustomization/pluginListWidget.ts:879

• Pass DOM.getWindow(this.cardScrollableNode) to this observer. The default is mainWindow, so plugin cards opened in an auxiliary editor window use a ResizeObserver from the wrong realm; the other observers in this widget already pass DOM.getWindow(this.element) (pluginListWidget.ts:704-708, 754-765).

		const cardResizeObserver = this._register(new DOM.DisposableResizeObserver(
			'PluginListWidget.cardScrollable',
			() => this.cardScrollable.scanDomNode(),
		));
		this._register(cardResizeObserver.observe(this.cardScrollableNode));

src/vs/workbench/contrib/chat/browser/aiCustomization/pluginListWidget.ts:1216

• This per-content observer also defaults to mainWindow. Since createCardScrollContent runs after the widget may have been mounted in an auxiliary window, pass DOM.getWindow(content) so dynamic card height changes are observed in the containing realm.

		const resizeObserver = this.cardDisposables.add(new DOM.DisposableResizeObserver(
			'PluginListWidget.cardScrollContent',
			() => this.cardScrollable.scanDomNode(),
		));
		this.cardDisposables.add(resizeObserver.observe(content));