Start autostart launches in tray

Add a shared --start-in-tray argument that lets autostart start the scheduler and tray integration without opening the main window.

Write the argument into Windows Startup shortcuts and Linux XDG Autostart desktop entries, and verify existing autostart entries include it.

Keep manual launches unchanged and let a manual second launch reveal an already-running instance while duplicate autostart launches stay hidden.

README.md

@@ -274,6 +274,7 @@ Standard 5-field cron schedules:
 Changing `jobs_dir` saves the current job list to the new directory.
 
 The `Start on login` setting shows an `OK` or `Problem` status next to the checkbox. Saving settings with the checkbox enabled rewrites the autostart entry using the current executable path.
+Autostart entries add `--start-in-tray`, so scheduled jobs begin running after sign-in without opening the main window.
 
 ## Autostart
 
@@ -292,7 +293,7 @@ Linux:
 [Desktop Entry]
 Type=Application
 Name=PySentry
-Exec=/opt/pysentry/pysentry-0.2.4-linux-amd64
+Exec=/opt/pysentry/pysentry-0.2.4-linux-amd64 --start-in-tray
 Terminal=false
 ```
 
@@ -301,8 +302,9 @@ Windows:
 ```text
 # PySentry writes a shortcut to the current user's Startup folder when Start on
 # login is enabled. A .lnk stores the executable path as a structured TargetPath,
-# so paths with spaces do not need fragile command-line quoting. Saving settings
-# rewrites the shortcut and removes old HKCU Run entries from earlier builds.
+# and stores --start-in-tray as Arguments, so paths with spaces do not need
+# fragile command-line quoting. Saving settings rewrites the shortcut and removes
+# old HKCU Run entries from earlier builds.
 %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup\GoSentry.lnk
 ```