Fix shellcheck errors and warnings

Fixes all shellcheck issues identified in the CI pipeline:

Critical Fix

• SC2168: Fixed local keyword used outside function (line 358) - moved variable declaration inside the conditional block

Code Quality Improvements

• SC2155 (23 occurrences): Separated variable declarations from assignments to properly handle return values and avoid masking errors
• SC2001 (2 occurrences): Replaced sed with bash parameter expansion for simple string substitutions (more efficient)
• SC2086: Added quotes around $PATH to prevent globbing
• SC2016: Fixed variable expansion in install.sh by using double quotes

All changes maintain the same functionality while following shellcheck best practices for safer and more maintainable shell scripts.