fix(web): smoother image viewing and live monitor updates

[DualChimerra]

• ImagePreviewModal renders through a portal to . It was mounted inside queue cards; finished cards have opacity .72, so the lightbox came out translucent, could sit under later cards and forced the whole page to be composited behind the full-size image every frame. Also locks body scroll, binds keydown once, and preloads the prev/next image.
• ZoomableImage keeps the current image painted while the next one loads (no blank flash when flipping), decodes async, refits in a layout effect.
• useMonitorProgress is now a shared per-task store: Topbar, Sidebar, Queue, QueueDetail and MonitorDashboard used to each fetch the full multi-MB snapshot and merge every 1s delta into their own 50k-point arrays.
• TaskSampleStrip and the dashboard's SampleViewer are memoized, so the per-second training ticks no longer re-render them (or an open lightbox).
• MonitorDashboard: chart series are memoized; fixes the learning-rate chart being rendered twice and the "d" chart floating outside a card - it now shows in its own card only when the optimizer reports d.