fix: standardize shebangs, error handling, and minor issues in x-* scripts

Normalize shebangs to #!/usr/bin/env bash (x-env-list, x-localip).
Use XDG_CONFIG_HOME in x-change-alacritty-theme. Remove unused
VERBOSE variable in x-multi-ping. Add set -euo pipefail to x-when-down
and x-when-up. Add usage header to x-term-colors. Fix notify-call
to notify-send.sh in x-record.

local/bin/x-multi-ping

@@ -39,7 +39,6 @@
 # Defaults
 LOOP=0
 SLEEP=1
-VERBOSE=0
 TIMEOUT=5
 
 usage()
@@ -60,8 +59,6 @@ while [[ $# -gt 0 ]]; do
       exit 0
       ;;
     --verbose)
-      # shellcheck disable=SC2034
-      VERBOSE=1
       shift
       ;;
     --loop | --forever)