feat(autostart): suppress browser auto-open on Windows login

When the user enables "Start with Windows" in the installer, the app
launches on every PC login. Previously each login popped a fresh WebUI
tab, which is noisy for a tray-resident background service.

The autostart shortcut now passes --autostart to start-hidden.vbs, which
sets LEDGRAB_AUTOSTART=1 in the child env. __main__ checks this flag
alongside LEDGRAB_RESTART when deciding whether to open the browser.

Manual launches (desktop/start-menu shortcuts) and the installer's
post-install "Launch LedGrab" finish-page action are unchanged — they
don't pass the arg, so they still open the WebUI tab.

server/src/ledgrab/__main__.py

@@ -83,6 +83,16 @@ def _is_restart() -> bool:
     return os.environ.get("LEDGRAB_RESTART", "") == "1"
 
 
+def _is_autostart() -> bool:
+    """Detect if launched via the Windows autostart shortcut."""
+    return os.environ.get("LEDGRAB_AUTOSTART", "") == "1"
+
+
+def _should_skip_browser() -> bool:
+    """Skip auto-opening the browser on restarts and on Windows login autostart."""
+    return _is_restart() or _is_autostart()
+
+
 def _check_port(host: str, port: int) -> None:
     """Exit with a clear message if the port is already in use."""
     with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
@@ -120,8 +130,8 @@ def main() -> None:
         )
         server_thread.start()
 
-        # Browser after a short delay (skip on restart — user already has a tab)
-        if not _is_restart():
+        # Browser after a short delay (skip on restart and on Windows login autostart)
+        if not _should_skip_browser():
             threading.Thread(
                 target=_open_browser,
                 args=(config.server.port,),