Linux desktop tray / panel applet support

[acamarata]

Summary

PrayCalc v1.1 shipped desktop tray support for Windows and macOS. Linux (GNOME AppIndicator, KDE StatusNotifierItem) was deferred from S19 due to DE fragmentation and limited tester availability.

Feature

• Prayer time countdown in GNOME top bar and KDE system panel
• Adhan notification via libnotify / org.freedesktop.Notifications
• No new feature flag needed — desktopTray flag already covers this
• Build target: flutter build linux --release

Acceptance Criteria

• Works on Ubuntu 22.04 LTS (GNOME Shell + AppIndicator)
• Works on Fedora 40 (GNOME)
• Works on KDE Neon (Plasma, StatusNotifierItem)
• CI passes flutter build linux --release
• featureFlagsProvider.desktopTray == true enables the tray icon

Implementation Notes

• Consider tray_manager package (cross-platform, includes Linux via AppIndicator)
• DBus dependency via dbus Dart package for libnotify
• Reference: Windows/macOS tray implementation in lib/features/desktop/

Deferred From

Sprint S19 T44 — PrayCalc v1.1