Fix string identity comparison bug in balance.py menu handling

[Copilot]

String comparisons in balance.py used is (identity check) instead of == (equality check), causing undefined behavior when matching user input against menu options.

Changes

• balance.py: Replace is with == for string comparisons (lines 73, 83, 84)

# Before
if option is '1':
    ...
elif option is '3':
    ...
elif option is '4':
    ...

# After  
if option == '1':
    ...
elif option == '3':
    ...
elif option == '4':
    ...

The is operator checks object identity, not value equality. While string interning may make this work for literals in the REPL, it fails unpredictably with user input from input().

Original prompt

Reference: 694c574

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