MAINT-11: start or stop the maintenance cycle per battery

maint_start and maint_abort take an optional unit. With two or more batteries
each Batteries card has Start cycle and Stop cycle buttons. One slot per site
stays: a start while a battery cycles is refused and names it (no queue). A
battery that is switched off, an unknown unit, and a stop on a battery that is
not cycling are refused with a plain message. A request with no unit is the
old call. The scheduler and the phase machine are unchanged.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bdqr7PiaHcBnzPituu2yTC

goodwe_controller/DOCS.md

@@ -3413,6 +3413,11 @@ day's price runs.
 An Off battery that the peak limit still needs says so, with the power it
 gives: "Off, but gives 0.5 kW for the peak limit."
 
+With two or more batteries, each card also has **Start cycle** and **Stop
+cycle**, to run the monthly cycle on that battery now. Only one battery cycles
+at a time: a start while another battery is cycling, or on a battery that is
+switched off, is refused and the message names the battery.
+
 **Settings** opens on the **Simple** view, or on the view you last picked:
 the peak limit, the batteries' switches and modes, and shadow mode, each
 marked "Applies right away" or "Applies after a restart". **Installer** shows