Resizable: implement a real resizable panel group #67

Neither the package Resizable nor its CLI template resizes anything: both just render a static flex layout with a decorative handle. Found while reconciling drift in #52 (recorded as allowed drift in TemplateSyncTests).

Should match shadcn's ResizablePanelGroup / ResizablePanel / ResizableHandle:

• horizontal and vertical directions
• dragging the handle with a pointer, plus keyboard arrows on the focused handle (role="separator", aria-valuenow)
• DefaultSize, MinSize and MaxSize per panel, in percent
• a layout-changed callback
• pointer tracking in shellui.js (package and ShellUIJsTemplate), using the same window.ShellUI path the other templates use

Once both copies match, remove resizable from AllowedDrift.