Add GNOME 50 support and fix deprecated GTK4 widgets

[kongvut]

Summary

• Add GNOME 50 to shell-version in metadata.json
• Fix DateMenu clearButton index for GNOME 49+ (DND button was removed, index changed from 1 to 0)
• Add try-catch and null guards for DateMenu widget tree traversal to handle future internal changes
• Add safety guards for private API access (_bannerBin, _bgManagers, panel boxes, _calendar)
• Replace all deprecated GTK4 widgets in prefs.js with modern equivalents

Changes

extension.js

• DateMenu: fixed clearBtn = msgHbox.get_child_at_index(1) → get_child_at_index(0) for GNOME 49+
• Wrapped DateMenu tree traversal in try-catch with ?. null guards
• Added null checks for Main.messageTray._bannerBin (3 locations)
• Added ?. guard for Main.layoutManager._bgManagers
• Added .filter(Boolean) for panel._leftBox/_centerBox/_rightBox
• Added null guard for Main.panel.statusArea?.dateMenu?._calendar

prefs.js

• Gtk.MessageDialog → Adw.MessageDialog
• Gtk.FileChooserDialog → Gtk.FileDialog (async open/save API)
• Gtk.ColorButton → Gtk.ColorDialogButton (signal: color-set → notify::rgba)
• Gtk.FontButton → Gtk.FontDialogButton (signal: font-set → notify::font-desc, added Pango import)
• Gtk.ComboBoxText → Gtk.DropDown + Gtk.StringList with manual index↔id mapping
• get_style_context().add_class() → add_css_class()

metadata.json

• Added "50" to shell-version array

Test plan

• Load extension on GNOME 50 — verify no crashes on startup
• Open DateMenu — verify notifications, calendar grid, clear button styled correctly
• Open prefs window — verify all combo boxes, color buttons, font button work
• Test import/export settings via file dialog
• Test reset settings dialog
• Verify panel styling applies correctly (bar, menus, dash)
• Check for no console errors in journalctl /usr/bin/gnome-shell

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