feat(toast): port upstream close buttons (#2023)

[tyulyukov]

Summary

Ports upstream #2023 toast refactor — adds visible X close buttons, ToastBodyContent extraction, and expandable body fields (expandableContent, expandableLabels, expandableDescriptionTrigger).

Conflicts resolved

• apps/web/src/components/ui/toast.tsx: Took upstream's version wholesale (492 → 719 lines). Upstream already re-integrated our CopyErrorButton (line 93) on error toasts, so nothing to hand-merge there.
• apps/web/src/components/Sidebar.tsx: Preserved MarCode's richer "Delete anyway" warning toast in SidebarProjectItem — action button, deferred close, api.dialogs.confirm(...) with thread-count messaging, removeProject(..., { force: true }), and inline error toast on failure. Migrated the outer toast calls to the new stackedThreadToast(...) helper for layout consistency with the rest of the file and the error-toast path that upstream already migrated.

Why this is isolated

Dedicated branch on top of origin/main — intentionally not mixed into PR #66 because:

1. toast.tsx is a large refactor (+227 net lines) and deserves its own review window.
2. The Sidebar "Delete anyway" preservation is a MarCode-specific behavior the reviewer should confirm on its own.

Test plan

• Local: bun run typecheck clean
• Local: apps/web full test suite — 1088/1088 passing (incl. toast.logic.test.ts 10/10)
• Local: oxlint — 0 errors (67 pre-existing warnings)
• Manual smoke:
  • Info / success / warning / error toast all render with an X close button
  • Clicking X dismisses the toast
  • Long error messages still show the Copy button + expandable body
  • Right-click → Delete project:
    • Empty project → confirms + removes
    • Non-empty project → shows "Delete anyway" warning; clicking it opens confirm dialog with thread count; Cancel aborts; OK deletes threads and project
    • Delete failure → stacked error toast
• FEATURES.md checklist — no MarCode exclusive feature regresses