fix: add budget entry input validation

[zhihuangzheng22-spec]

Implemented input validation for budget entries.

Changes:

• Added validateEntryInput() to check title and amount
• Rejected blank titles and titles containing only spaces
• Rejected zero, negative, and invalid amounts
• Reused the same validation helper for both income and expense entries
• Added validation messages for English, Chinese, and Japanese

Testing:

• Empty title is rejected
• Space-only title is rejected
• Amount 0 is rejected
• Negative amount is rejected
• Valid income and expense entries can still be added normally
• Language switching still works

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