feat: Add undo delay on destructive actions (cancel stream)

[Austinaminu2]

Description

Cancelling a stream is irreversible — the sender gets unstreamed tokens back and the stream is permanently ended. Currently, the app shows a confirmation dialog, but once confirmed, the transaction is submitted immediately.

Adding an undo delay (similar to Gmail's "Undo Send") would give users a safety net against accidental cancellations.

Proposed Solution

After the user confirms cancellation:

1. Show a 10-second countdown toast: "Stream will be cancelled in 10s — [Undo]"
2. If the user clicks "Undo", the cancellation is aborted
3. If the countdown expires, submit the cancel transaction
4. During the countdown, the stream card shows a "Cancelling..." state

This pattern should also apply to other destructive actions if added in the future.

Acceptance Criteria

• Countdown toast appears after confirming stream cancellation
• "Undo" button in the toast aborts the cancellation
• Countdown duration is configurable (default 10 seconds)
• Stream card shows a "Cancelling..." visual state during countdown
• If user navigates away during countdown, cancellation is aborted (safe default)
• Successful cancellation shows the normal success toast after countdown