fix: remove double-pop from BuildingTransactionDialog cancel

[sneurlax]

The dialog's cancel button popped the navigator itself and then invoked onCancel, while callers' onCancel handlers also popped, causing a double pop. Drop the internal pop so the caller owns the dismissal, and add the missing pop to Step4View's onCancel.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.84%. Comparing base (8208fe7) to head (b21c190).
⚠️ Report is 11 commits behind head on staging.

Files with missing lines	Patch %	Lines
..._view/sub_widgets/building_transaction_dialog.dart	0.00%	10 Missing ⚠️
...exchange_view/exchange_step_views/step_4_view.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           staging    #1364   +/-   ##
========================================
  Coverage     4.84%    4.84%           
========================================
  Files         1092     1092           
  Lines       109141   109133    -8     
========================================
  Hits          5283     5283           
+ Misses      103858   103850    -8     

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