Add configurable window position setting

[mircea-pop]

Summary

Adds a Window Position preference to the Settings dialog with two options:

• Center (default) - existing behavior, opens centered on the primary monitor
• Mouse Cursor (Gtk.WindowPosition.MOUSE) - opens on the monitor where the mouse cursor currently is

Problem: On multi-monitor setups, the picker always opens centered on the primary monitor, even when the user is working on a different screen.

Scope: This PR ensures the picker opens on the correct monitor (where the mouse is). Note that on Wayland, the compositor controls exact window placement - Gtk.WindowPosition.MOUSE tells GTK which monitor to use, but the compositor (e.g. Mutter) determines the exact position within that monitor. Precise cursor-following positioning would require Wayland-specific APIs (e.g. xdg-positioner or layer-shell) which is outside the scope of this change.

Follows existing patterns for settings (theme, skintone) - stored via shelve in user_data.py, exposed as a combo box in settings.py.

Changes

• emote/user_data.py - window position constants, load_window_position(), update_window_position()
• emote/settings.py -"Window Position" combo box in Preferences
• emote/picker.py - use configured position instead of hardcoded CENTER

How was tested

• Open Emote with default settings → picker opens centered (no behavior change)
• Open Preferences → change Window Position to "Mouse Cursor" → close Preferences
• Open Emote again → picker opens on the monitor where the cursor is
• Restart Emote → setting persists
• Test on multi-monitor setup (4 monitors): picker appears on the correct monitor