Cascade 0.9.2: shareable editors on phones

Added

• Exported editors work on phones. The preview is full width at the top,
  with pinch, pan and double-tap to fit. Controls sit in a bottom sheet with
  three snap points and folded groups, sized for thumbs (44 px targets, 16 px
  text, steppers). The header is one row with a ⋯ menu. Safe-area insets and
  100dvh keep everything clear of Safari's toolbar. Exports and scenes use the
  share sheet, with a download as the fallback. Landscape uses a side panel, and
  desktop is unchanged.
• editor.peek and editor.draft. These choose the controls kept in reach
  at the sheet's lowest point, and the values cooked while a slider is dragged
  (a lower density, say).