Fix #316: add Linux-native tray fallback and notifications when AWT SystemTray is unavailable

[QuickMythril]

Summary

Fixes #316 by making Qortal Core tray support work on Linux systems where java.awt.SystemTray is unsupported.

Root Cause

The existing tray implementation was AWT-only.
On many Linux desktop setups, SystemTray.isSupported() returns false, so tray initialization exited early and no icon/menu/notifications were available.

What This PR Changes

1) Add Linux-native tray backend fallback

• Added com.dorkbox:SystemTray dependency in pom.xml.
• SysTray now supports backend selection via:
  • -Dqortal.tray.backend=auto|awt|sni|appindicator|none
• Default auto behavior:
  • Try AWT tray first (preserves current behavior where it works)
  • On Linux, fall back to Linux-native tray if AWT is unavailable

2) Improve backend diagnostics

• Added startup logging for tray backend decisions:
  • requested backend
  • OS
  • headless state
  • AWT support
  • session/desktop env values

3) AppIndicator tooltip fallback

• AppIndicator often does not support hover tooltips.
• For Linux-native AppIndicator, tray text now falls back to status text (setStatus) instead of tooltip only.

4) Linux notification support

• Linux-native backend now attempts desktop notifications via notify-send from showMessage(...).
• If notify-send is unavailable/fails, it gracefully falls back to debug logging (no crash/regression).

Compatibility / Risk

• Windows/macOS and Linux systems where AWT tray already works keep existing behavior.
• Linux-native path is only used when selected or when auto fallback is needed.
• No API changes to existing tray callers.

Commits Included

1. 65fb8cfd feat(gui): add Linux-native system tray backend fallback
2. 3cfc1254 fix(gui): fallback to status text for AppIndicator tray
3. 139a3798 feat(gui): send Linux tray notifications via notify-send

Validation

• Project compiles successfully (mvn -DskipTests compile).
• Manual Linux verification with appindicator:
  • tray icon appears
  • tray menu works
  • status text updates
  • desktop notifications are shown