Add GNOME 50 Support: migrate GTK3 widgets to GTK4, add null-saftey gaurds, update schema for accent color

[jasonpiedrasantasdk]

Adds support for GNOME Shell 50 (currently 50.1) with comprehensive GTK4 updates.

Changes:

• Add "50" to shell-version in metadata.json, bump version to 50
• Guard Gio._promisify calls (removed in GJS 1.82/GNOME 50)
• Null-safety fixes: add optional chaining (?.) in extension.js where objects may be null
• Migrate prefs.js to GTK4 widgets:
  • Gtk.ComboBoxText → Gtk.DropDown + Gtk.StringList with custom _optionIds tracking
  • Gtk.ColorButton → Gtk.ColorDialogButton with Gtk.ColorDialog
  • Gtk.FontButton → Gtk.FontDialogButton + Pango.FontDescription
  • Gtk.MessageDialog → Gtk.AlertDialog (async choose pattern)
  • Gtk.FileChooserDialog → Gtk.FileDialog (async open/save pattern)
  • get_style_context().add_class() → add_css_class()
• Update GSConnect compatibility: GLib < 2.80 fallback for GioUnix
• Add use-system-accent schema key for accent color theming

Credits:

• GTK4 widget migration (DropDown, FontDialogButton, ColorDialogButton,
  AlertDialog, FileDialog) adapted from kongvut's work in upstream PR Add GNOME 50 support and fix deprecated GTK4 widgets #171
• Gio._promisify guard pattern from multiple community sources
• Null-safety fixes and general GNOME 50 audit by @jasonpiedrasantasdk