Summary

Rework the in-builder preview to render as a fixed overlay instead of replacing
the canvas, eliminating layout shifts and iframe rebuilds on every toggle.

Changes

• Render preview as a fixed overlay above the canvas (no sidebar/canvas resize)
• Add independent zoom state for preview (separate from canvas zoom)
• Extract shared ViewportZoomControls component for both canvas and preview toolbars
• Add iframeRef support to useZoom hook for wheel zoom inside iframes
• Add shortcutsEnabled flag to useZoom to toggle keyboard/wheel handlers per mode
• Autofit preview on open and on breakpoint switch
• Scroll canvas to selected layer on breakpoint change with smooth animation
• Show YCode logo with shimmer text as preview loading indicator
• Add "Open in browser" button to preview toolbar
• Add external-link icon to Icon component
• Fix zoom dropdown width for consistent layout

Test plan

• Toggle preview mode — canvas should not resize or re-render
• Cmd+wheel zoom works in both canvas and preview (independently)
• Cmd+0/1/2 shortcuts only affect the active mode (canvas or preview)
• Switching breakpoints in preview autofits and does not reload iframe
• Switching breakpoints on canvas with a selected layer scrolls to it
• "Open in browser" button opens the page in a new tab
• Loading state shows YCode logo with shimmer text