Use minimum effective cancellation time for KWR refunds (OZ-5)

[adnanhq]

Changes

PausableCancellable

• Added uint256 private _cancellationTime storage variable
• In _cancel(), added _cancellationTime = block.timestamp so every cancellation records when it happened
• Added cancellationTime() public getter

KeepWhatsRaised

• Removed uint256 private s_cancellationTime — redundant now that PausableCancellable tracks it
• Removed s_cancellationTime = block.timestamp from cancelTreasury() — _cancel() handles this
• Added _getEffectiveCancellationTime() — checks both the treasury’s own cancellationTime() (inherited)
  and PausableCancellable(address(INFO)).cancellationTime() (campaign’s), returning the earlier of the two
  if both are set, or whichever is non-zero
• Updated _checkRefundPeriodStatus(), claimTip(), and claimFund() to use the effective cancellation time
  instead of the local-only s_cancellationTime

This ensures that when a campaign is cancelled (even without explicitly cancelling the treasury), refund
scheduling uses the campaign’s cancellation timestamp — backers get immediate access to refunds rather
than waiting for the original deadline.

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