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/packmode.py

@@ -42,6 +42,12 @@ MODE_KEY = "pack_mode"
 SELL_KEY = "pack_sell"              # reserved for P3
 
 
+def enabled(opts) -> bool:
+    """Battery modes are on only when the option is exactly true. One answer,
+    read by the controller and by the MQTT announcement."""
+    return opts.get("battery_modes_enabled") is True
+
+
 @dataclass(frozen=True)
 class PackMode:
     """One battery's choice: on or off, its mode, and (P3) selling."""