fix: harden settings screen values against NullPointerException (#413)

[Himanth-reddy]

Summary

Fixes issue #413 where opening the Settings screen consistently crashes with a NullPointerException: Attempt to invoke interface method 'int java.lang.CharSequence.length()' on a null object reference.

Root Cause

When opening the Settings screen or switching locales (such as es-ES), setting strings or preference values loaded from DataStore/SharedPreferences or resource lookups can pass null into UI row composables and formatters.
Previously, functions such as SettingsRow, MobileSettingsRow, localizeSettingValue, and formatUserAgentPreview expected non-null String parameters and invoked operations like .isNotBlank(), .isNotEmpty(), or .ifBlank {} on them directly, causing CharSequence.length() to be called on a null reference.

Changes Made

• SettingsRows.kt:
  • Updated localizeSettingValue(value: String?) and SettingsRow(..., value: String?) to safely handle nullable string inputs, defaulting to empty strings before evaluating length/emptiness.
• SettingsScreen.kt:
  • Updated formatUserAgentPreview(value: String?, maxLength: Int) and MobileSettingsRow(..., value: String?) to accept nullable values and evaluate safe fallback strings via .orEmpty().

Resolves #413.