iOS can orphan a billed cube when New Session is dismissed during provisioning #312

Summary

Dismissing the New Session sheet while cube provisioning is in progress does not cancel the provisioning task or roll back a sandbox it has created. Provisioning can continue after the UI disappears, incur a full spin-up, and finish without handing the resulting session back to the Sessions list.

Reproduction

1. Sign in and open New Session.
2. Enter a valid prompt and tap Start building.
3. While the UI is in the launching phase, dismiss the sheet.
4. Observe gateway activity/sandbox inventory: the untracked launch continues and can create/start/configure a cube.
5. Return to the Sessions list; no session is handed off because dismissal occurred before .chat.

Root cause

• The Start button creates an untracked Task { await launch() } (apps/ios/Graff/Sources/NewSessionView.swift:46-51).
• The task is not retained for cancellation.
• .onDisappear { finish() } runs on dismissal, but finish() only calls onCreated when phase == .chat (NewSessionView.swift:111-121). During .launching it simply dismisses.
• CubeBroker.launch can continue creating, starting, installing, provisioning, and launching services after the view is gone.

Expected behavior

• Retain and cancel the provisioning task when the user cancels/dismisses.
• Make the launching UI's cancellation behavior explicit.
• If this flow created a new sandbox, stop/delete or otherwise record it on cancellation.
• If cancellation arrives after a usable connection exists, persist/hand off a recoverable session rather than orphaning it.
• Add a delayed-provisioning dismissal test.