Improve variable and function naming for clarity

[Copilot]

Renamed ambiguous variables and constants throughout the codebase to improve readability and self-documentation.

Duration Constants

• Const.day/week/year → Const.dayInMilliseconds/weekInMilliseconds/yearInMilliseconds

Utils Module

• monthNum → monthNumber
• monthLength → daysPerMonth
• _leapYear() → _isLeapYear()
• leap → isNonLeapCentury

Runner Module

• onceKey → versionKey
• buildKey → buildNumberKey
• existingVersion/existingBuild → parsedCurrentVersion/parsedCurrentBuild

All references updated across runner.dart, runner_once.dart, and builder_once.dart.

Original prompt

Suggest more descriptive variable and function names

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