Add complex mode

Complex mode for mathematical computation in complex numbers. (not to be confused with complex number support.

The issue mentions adding a complex mode for mathematical computation, but does not specify what changes are needed. Look at the existing calculator code to understand the current arithmetic logic and UI. Determine where complex number handling would be integrated, likely in the core calculation engine and possibly the UI for input/output. Check if there are any tests for the calculator's mathematical functions.

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