Updated `getSignatureNavState`, `goToNextSignature`, and
`goToPreviousSignature` methods to enhance code readability
by improving comments and clarifying logic. Fixed a minor bug
in `goToPreviousSignature` where the wrong variable (`lastSig`)
was used for page navigation, replacing it with the correct
variable (`prevSignature`). Ensured `_lastViewedSignatureId`
is updated correctly in navigation methods. No significant
functional changes were introduced.