osd: add multi-monitor support

osd: add multi-monitor support

Adds `output` attribute to control which monitor(s) display the window
switcher OSD. Supports three modes:
- "all": display on all monitors (default)
- "pointer": display on monitor with mouse cursor
- "keyboard": display on monitor with keyboard focus

The configuration structure is also refactored to nest OSD-specific
settings (show, style, output, thumbnailLabelFormat) under an <osd>
element within <windowSwitcher>, improving logical organization.