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/app/messages.py

@@ -1022,6 +1022,21 @@ def pack_refused(why: str) -> str:
     return PACK_REFUSED[why]
 
 
+# MAINT-11: why a start or stop of ONE battery's cycle was refused. `{unit}` is
+# the battery the sentence is about (for "busy", the one that is cycling). The
+# page shows the same sentences in Dutch from app/ui_text, keyed by the code.
+MAINT_REFUSED = {
+    "unit": "There is no battery with that name.",
+    "busy": "Only one battery can cycle at a time, and {unit} is cycling now.",
+    "off": "{unit} is switched off, so it cannot start a cycle.",
+    "idle": "{unit} is not running a cycle.",
+}
+
+
+def maint_refused(why: str, unit: str = "") -> str:
+    return MAINT_REFUSED[why].format(unit=unit)
+
+
 def hint(key: str, **kw) -> str:
     """The plain words for one event. `w` is formatted as watts, `sw` as
     watts with their sign (positive gives power, negative charges)."""