Fix CSS snippet creation and cleanup in callout manager

[ebullient]

Changes

• Improved snippet management: Enhanced setUseSnippet() to properly disable and remove CSS snippets when the feature is toggled off
• Fixed snippet directory creation: Added logic to create the snippets directory if it doesn't exist before writing the snippet file
• Simplified write operation: Unified file creation and update into a single write() call for cleaner code
• Updated documentation: Added note to settings that CSS snippet is required for correct rendering in popout windows

Fixes

• Prevents errors when snippet directory structure doesn't exist
• Ensures proper cleanup when disabling the snippet feature
• Better error handling for file removal