feat(watchdog): separate watchdog from startup

- Add /api/v1/watchdog to enable/disable
- Scheduled task now runs every 15 minutes
- Use schtasks command line instead of XML
- Dashboard shows watchdog enabled state
- Preserve watchdog state during install

lib/startup/trcmdline.go

@@ -0,0 +1,23 @@
+package startup
+
+import (
+	"fmt"
+	"strings"
+)
+
+const ensureArg = "-ensure"
+
+// ensureTaskTR is the schtasks /TR task action: "<exe> -ensure".
+func ensureTaskTR(exe string) string {
+	return strings.Trim(exe, `"`) + " " + ensureArg
+}
+
+// schtasksCreateCmdLine is the full Windows command line for schtasks /Create.
+// ponytail: SysProcAttr.CmdLine bypasses Go's /TR quoting bugs.
+func schtasksCreateCmdLine(taskName, tr string) string {
+	return fmt.Sprintf(
+		`schtasks.exe /Create /TN %s /SC MINUTE /MO 15 /RL LIMITED /F /TR "%s"`,
+		taskName,
+		tr,
+	)
+}