Add File Manager as a configurable Defaults category

[oscarbol09]

Browser, Terminal, Editor, and Agent all have dedicated categories under Setup > Defaults with dynamic launcher scripts that resolve the user's configured default. File Manager was previously hardcoded to Nautilus in both the bindings and commands.

This adds File Manager as a first-class Defaults category:

• Adds bin/omarchy-default-filemanager to get and set the default file manager via xdg-mime, supporting Nautilus, Dolphin, Thunar, Nemo, and PCManFM.
• Adds bin/omarchy-launch-filemanager to resolve and launch the configured default file manager via uwsm-app.
• Adds bin/omarchy-launch-filemanager-cwd to launch the default file manager in the active terminal's current directory.
• Adds setup.default.filemanager entries to default/omarchy/omarchy-menu.jsonc.
• Updates Hyprland SUPER + SHIFT + F and SUPER + ALT + SHIFT + F bindings to launch the configured default file manager.

Fixes #10478