fix(GUI): DrawScrollBar track paging + edge grab + blocked dim

Card eb45ab (analyst: all sites confirmed).

• Track paging: isHoveringTrack (same Blocked + IsPointInClip gates) + click pages toward the pointer by viewRatio — was dead before
• Pressed-edge grab: mouse['Pressed']['Left'] instead of HELD — sliding onto the thumb mid-drag no longer grabs it; ConsumeClick on both grab + track-page stops the leak-through
• Blocked dim: thumb face at GuiConfig.DisabledOpacity (#1335 convention) instead of full-strength dead affordance