🧹 Refactor duplicated systemd user service reversion logic

[raythurman2386]

This change addresses a code health issue in lib/functions.sh where the logic to revert systemd user services was duplicated across revert_swaybg, revert_waybar, and revert_mako.

Key changes:

• Introduced remove_user_service in the "Service Management" section of lib/functions.sh.
• Refactored revert_swaybg, revert_waybar, and revert_mako to use this new helper.
• Added a defensive check for the existence of the service file before performing removal operations.
• Maintained existing behavior of ignoring errors during service stop/disable and performing a daemon-reload.

Verification:

• Ran bash -n lib/functions.sh to confirm syntax.
• Ran bash validate.sh to ensure all core scripts and configurations are valid.
• Visually inspected the refactored code for correctness.

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PR created automatically by Jules for task 13706707406658816868 started by @raythurman2386

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