fix(settings): add SFM Forward tab to settings dialog (v1.5.1)

v1.5.0 shipped the forwarder module + INI keys but the settings
dialog wasn't extended, so the only way operators could enable
forwarding was hand-editing config.ini.  This adds a sixth tab
("SFM Forward") with:

  - Forward events to SFM    checkbox
  - SFM Server URL           entry + Test button (GETs /health)
  - Forward Interval (sec)   spinbox
  - Quiescence (sec)         spinbox
  - Missing-Report Grace     spinbox
  - HTTP Timeout             spinbox
  - State File               entry + Browse...

Save-time guard: enabling the forwarder without a URL raises a
validation error rather than silently saving a non-functional
config.

Patch release — same on-disk INI schema, no config migration.

series3_tray.py

@@ -1,5 +1,5 @@
 """
-Series 3 Watcher — System Tray Launcher  v1.5.0
+Series 3 Watcher — System Tray Launcher  v1.5.1
 Requires: pystray, Pillow, tkinter (stdlib)
 
 Run with:  pythonw series3_tray.py   (no console window)