feat: add focus-follows-mouse option

[jimsynz]

Summary

• Add new focus_follows_mouse configuration option that automatically focuses panes when the mouse hovers over them
• Integrates seamlessly with existing mouse hover detection system
• Requires advanced_mouse_actions to be enabled

Configuration

Works via both config file and CLI:

Config file:

options {
    advanced_mouse_actions true
    focus_follows_mouse true
}

CLI:

zellij options --focus-follows-mouse true

Implementation Details

• Defaults to false for backwards compatibility
• Leverages existing hover detection in tab/mod.rs
• Uses existing focus_pane_with_id() method for consistency
• Extends MouseEffect struct with focus_changed field
• Minimal implementation (~100 lines of code)

Test plan

• All existing tests pass
• New configuration option appears in generated config files
• CLI flag works correctly with options subcommand
• Feature integrates properly with existing mouse system
• Backwards compatible (defaults to disabled)