Phase 6: design polish — accept confetti + distraction-free mode

- Accept confetti: CSS-only 4-dot burst spawned at the accepted card,
  trajectory via inline --dx/--dy, auto-cleared after 720ms.
- Distraction-free mode: editor focus collapses the doc-list sidebar
  (width→0 slide), canvas re-centers full-width; Escape or a click
  outside the canvas (header/gutter/status bar) restores it.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/components/DocList/DocList.tsx

@@ -13,7 +13,7 @@ interface Props {
 export function DocList({ docs, selectedId, onSelect, onCreate, onDelete }: Props) {
   return (
     <aside
-      className="flex w-[260px] shrink-0 flex-col gap-2 p-3"
+      className="flex h-full w-[260px] flex-col gap-2 p-3"
       style={{ borderRight: '1px solid var(--color-border)' }}
     >
       <button