playground sidebar: reflow canvas + smooth drag-to-open

Created by: jedmund

Summary

• The playground's side configurator panel now reflows the canvas instead of overlaying it. Exercises and tools center in the remaining space (viewport minus panel). Docked keyboards are the one exception: they stay edge-to-edge even when the panel is open.
• The chevron handle can now be dragged open/closed, including via edge-swipe from the bezel. Tap still toggles. Drag tracks the finger; a flick (~200pt/s) commits; midpoint release snaps to the nearer edge.
• Fixed a ~10pt drag wobble caused by the gesture measuring translation in its own local frame — which was itself being translated by the offset the gesture produced. Anchoring to .global breaks the feedback loop.

Implementation notes

• SidePanelMetrics.panelWidth is the single source of truth for the panel's trailing inset.
• PlaygroundScreen carves out the trailing edge with .safeAreaPadding(.trailing:) on its content region and publishes the inset to descendants via EnvironmentKey (PlaygroundSidebarInsetKey).
• ScaleBuilderExercise reads the env inset and applies a negative trailing padding on its docked keyboard region to cancel the ancestor inset, keeping the keyboard full-width while the card above shrinks.
• Merged the previous .onTapGesture + DragGesture pair into a single DragGesture(minimumDistance: 0) that dispatches tap vs. flick vs. midpoint in .onEnded, eliminating tap/drag race conditions.

Test plan

• Home → Keyboard (contained): open panel, keyboard recenters in the left region; close, recenters in full viewport.
• Home → Keyboard (docked): open panel, docked keys stay edge-to-edge; play button / labels above shift left.
• Oscillator / Shaper / Interval tools: open panel, card shifts left smoothly.
• Scale Builder (playground): open panel, glass card shifts left; docked keyboard stays edge-to-edge.
• Scale Builder (lesson, 2-ex4.json): no panel, unchanged.
• Tap chevron: toggles open/closed with spring animation.
• Edge-swipe from right bezel (panel closed): drags panel open under finger.
• Slow-drag handle near midpoint: handle pixel-locks to finger, no wobble.
• Flick left then release: commits open even if release is short of midpoint.
• Release at midpoint with no velocity: snaps to nearer edge.
• Scroll configurator content vertically while panel is open: scrolls normally.
• Navigate away with panel open: slides closed in sync with nav animation.