UX-15: setup-only mode, and the wizard fixes F1, F2, meter, step 1

A new install names no inverter. When that is the startup gate's only
refusal, the add-on now serves the setup wizard in setup-only mode instead
of exiting 78. It builds no controller, MQTT client or meter task, its
HomeAssistant is read-only (every service call refuses before sending),
Controller refuses a read-only HomeAssistant, and /action answers 409.
Any other refusal still exits 78. DECISIONS/UX-15-setup-only-mode.md.

Wizard fixes from the UX-13 P4 test:
- F1: /config/validate converts what it can, so a conversion error blocks
  its own step and the live checks still run; save 422 carries steps.
- F2: saves send the model the page opened (base); only keys this page
  changed are written onto the blob stored now.
- A meter entity missing from Home Assistant is red on the last step.
- Step 1 says the reading went down after a skip that followed a fall.

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

goodwe_controller/app/validate.py

@@ -808,6 +808,14 @@ class ConfigRefused(Exception):
         super().__init__(problems[0] if problems else "configuration refused")
 
 
+def no_power_request_problem() -> str:
+    """Rule 1's refusal: no inverter to control. UX-15's setup-only mode
+    starts when this is the only refusal (app/setuponly.py)."""
+    return (f"{S('setpoint_entity')} is empty, so the add-on cannot control "
+            "the inverter. Set it to the ESPHome number entity for the "
+            "setpoint.")
+
+
 def validate_startup_config(opts: dict, dev_min=None, dev_max=None,
                             notes: list | None = None) -> list[str]:
     """Return the list of reasons this configuration must not be started.
@@ -828,9 +836,7 @@ def validate_startup_config(opts: dict, dev_min=None, dev_max=None,
     # --- 1. an actuator to write to, and AT MOST ONE WRITER PER ACTUATOR -----
     writers = actuator_writers(opts)
     if not writers:
-        problems.append(
-            f"{S('setpoint_entity')} is empty, so the add-on cannot control the "
-            "inverter. Set it to the ESPHome number entity for the setpoint.")
+        problems.append(no_power_request_problem())
     # SAFETY-28. The §1.5 non-goal, graded on the writers rather than on the
     # count of option keys. Read `actuator_writers()` for why the old check went
     # green on the very config this one is here to refuse.
@@ -1704,6 +1710,14 @@ WATT_ENTITY_KEYS = (
     "p1_phase_export_entities", "batt_entity", "peak_forecast_entity")
 
 
+# The meter entities each `meter_source` reads through Home Assistant.
+METER_ENTITY_KEYS = {
+    "off": ("meter_entity",),
+    SOURCE_HA_SIGNED: ("p1_net_entity", "p1_phase_net_entities"),
+    SOURCE_HA: ("p1_import_entity", "p1_export_entity",
+                "p1_phase_import_entities", "p1_phase_export_entities")}
+
+
 def _entities_read(opts: dict) -> list[tuple[str, str, str]]:
     """(key, inverter name or "", entity id) for each entity the add-on reads
     with these options. Keys the chosen meter source or roster shape ignores
@@ -1716,12 +1730,7 @@ def _entities_read(opts: dict) -> list[tuple[str, str, str]]:
                 out.append((key, where, e.strip()))
 
     source = str(opts.get("meter_source") or "off").strip()
-    by_source = {"off": ("meter_entity",),
-                 SOURCE_HA_SIGNED: ("p1_net_entity", "p1_phase_net_entities"),
-                 SOURCE_HA: ("p1_import_entity", "p1_export_entity",
-                             "p1_phase_import_entities",
-                             "p1_phase_export_entities")}
-    for key in by_source.get(source, ()):
+    for key in METER_ENTITY_KEYS.get(source, ()):
         add(key, opts.get(key))
     entries = roster_entries(opts)
     if not entries:
@@ -1844,7 +1853,7 @@ def setup_checks(opts: dict, *, entities: dict | None, running=None,
     entity exists in Home Assistant (a write to a missing one is answered 200,
     SAFETY-77, so only the entity list can tell), the battery levels read, and
     prices are known. Tier "ok", "info", "warn", or "bad" for a missing power
-    request entity, which the wizard will not save. Reads nothing itself:
+    request or meter entity, which the wizard will not save. Reads nothing itself:
     `entities` is Home Assistant's list by id (None when it did not answer),
     `running` the options this process started with, `meter_now_w` its meter
     reading and `prices_known` how many of today's hours have a price.
@@ -1855,7 +1864,11 @@ def setup_checks(opts: dict, *, entities: dict | None, running=None,
         _advise(notes, "bad", ("setpoint_entity",), "Home Assistant is not "
                 "answering, so press Check again once it is back.")
     else:
-        _entity_notes(notes, opts, entities, bad=("setpoint_entity",))
+        # A missing meter entity is red too (UX-15): with the kettle test
+        # skipped, nothing else on the way to the save would catch it.
+        meter_keys = {k for keys in METER_ENTITY_KEYS.values() for k in keys}
+        _entity_notes(notes, opts, entities,
+                      bad=("setpoint_entity",) + tuple(sorted(meter_keys)))
     meter = _meter_check(notes, opts, entities, running, meter_now_w)
     batts = [_card_checks(notes, who, entry, entities)
              for who, entry in _cards(opts)]