OPT-11 P2: HA entities for battery modes

Each battery gets an MQTT-discovered Mode select and Use switch, only while
battery_modes_enabled is true. Commands go through handle_action(), the
/action path; refusals are logged once. Retained commands are never applied.
Turning modes off removes exactly the configs announced (kept in the store).
With modes off the announcement is byte-identical to ed739d4.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

goodwe_controller/app/messages.py

@@ -932,6 +932,11 @@ PACK_REFUSED = {
     "off": "Battery modes are off. Turn on “Battery modes” first.",
     "unit": "There is no battery with that name.",
     "value": "That choice is not possible for this battery.",
+    # OPT-11 P2: a Home Assistant command dropped before it reaches a battery.
+    "retained": "A saved command is not used after a restart.",
+    "size": "That command is too long.",
+    "encoding": "That command is not plain text.",
+    "topic": "That is not a battery command.",
 }