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/DECISIONS/OPT-11-battery-modes.md

@@ -17,5 +17,12 @@ P1 is its section 6 P1, and the pull request lists what was measured.
   capped grid charge release an Off battery. Both are graded in
   `test_arbiter.py` part 11. The loop bound leaves a side that reaches the
   rail to the rail, so SAFETY-03 still flags a loop winding into it.
+- P2 (HA entities): a select and a switch per battery, only while modes are
+  on; commands go through `handle_action()`, the page's path. A retained
+  command is dropped, not applied: the store already holds the last choice,
+  and a replay after a restart could otherwise undo a newer one. The config
+  topics announced are kept in the store (`mqtt_mode_configs`), so turning
+  modes off removes exactly those, and an install that never turned them on
+  publishes what ed739d4 did (`test_mqtt.py` part 10).
 - In P1 only Off closes a direction; Leftover solar differs by charging last.
   Price runs, selling (`pack_sell`, reserved) and the forecast are P3/P4.