Improve error handling: stop silently swallowing errors

[devin-ai-integration]

Summary

Several handlers in calc.py either ran eval()/float() with no error handling — so invalid or empty input raised exceptions that Tkinter's callback loop silently swallowed to stderr, leaving the UI unresponsive with no feedback — or caught errors without logging them. This PR makes those failures visible: the user sees "Error" and the exception is logged, instead of the app doing nothing or crashing.

Changes:

• square(), sqrt(), perc(): wrapped the eval() calls in try/except. On failure they now logger.exception(...) and set current_expression = "Error" (matching evaluate()'s behavior) instead of letting an uncaught SyntaxError/NameError escape. Previously e.g. pressing x² with an empty display crashed the callback (eval("**2") → SyntaxError).
• evaluate(): the except Exception as e: swallowed e without logging. Now logs via logger.exception(...) and also clears total_expression so a bad expression doesn't persist into the next evaluation.
• memoria() (MS/M+/M-): float(self.current_expression) on non-numeric input (e.g. "Error") raised an uncaught ValueError. Now the value is parsed once up front; on ValueError it logs a warning, shows "Error", and returns instead of crashing.
• Window icon load in __init__: tk.PhotoImage(file=...) raised an uncaught TclError and aborted startup if the icon was missing. Now caught and logged as a warning so the app still launches.
• Added a module logger and logging.basicConfig(...) in __main__ so these logs actually surface.

Behavior verified headlessly (xvfb): empty x²/√, M+ on "Error", and evaluating an invalid expression all resolve to "Error" without crashing.

Link to Devin session: https://app.devin.ai/sessions/af3f52009c97490cab8030809ab866ef
Requested by: @martlyma

[devin-ai-integration]

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