refactor(core): remove unused utility functions

Remove unused functions from base.sh and ui.sh:
- base.sh: is_interactive, spinner stack management, get_terminal_info, validate_terminal_environment
- ui.sh: with_spinner

Net reduction: ~166 lines of dead code

lib/core/ui.sh

@@ -387,20 +387,6 @@ stop_inline_spinner() {
     fi
 }
 
-# Run command with a terminal spinner
-with_spinner() {
-    local msg="$1"
-    shift || true
-    local timeout=180
-    start_inline_spinner "$msg"
-    local exit_code=0
-    if [[ -n "${MOLE_TIMEOUT_BIN:-}" ]]; then
-        "$MOLE_TIMEOUT_BIN" "$timeout" "$@" > /dev/null 2>&1 || exit_code=$?
-    else "$@" > /dev/null 2>&1 || exit_code=$?; fi
-    stop_inline_spinner "$msg"
-    return $exit_code
-}
-
 # Get spinner characters
 mo_spinner_chars() {
     local chars="|/-\\"