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/main.py

@@ -87,9 +87,11 @@ from .watchdog import (
     AGAINST_METER, AGAINST_OWN_POWER, SIGN_OPTION_DEFAULTS,
     SignWatchdog, SignWatchdogConfig, loop_is_following,
 )
+from . import setuponly
 from . import web
 
 OPTIONS_PATH = "/data/options.json"
+INGRESS_PORT = 8099
 _LOG = logging.getLogger("goodwe")
 
 # ⚠️ `UNIT_A` NOW LIVES IN app/inverter.py AND IS RE-EXPORTED HERE, because the
@@ -375,6 +377,11 @@ class Controller:
     def __init__(self, opts: dict, hass: HomeAssistant, store, mqtt_pub, *, stop,
                  peak_db_path: str = "/data/tariff.db",
                  price_history_path: str = "/data/price_history.json"):
+        # UX-15: a read-only HomeAssistant belongs to setup-only mode, which
+        # builds no controller. Refused here so no controller can hold one.
+        if getattr(hass, "read_only", False) is True:
+            raise ValueError("a controller cannot run on a read-only "
+                             "HomeAssistant (setup-only mode)")
         self.o = opts
         self.hass = hass
         self.store = store
@@ -6011,6 +6018,15 @@ async def amain() -> None:
             _dev_min, _dev_max, _ = await hass.limits(_sp)
             _LOG.info("setpoint device %s reports range %s…%s W", _sp, _dev_min, _dev_max)
         _problems = validate_startup_config(opts, _dev_min, _dev_max)
+        # UX-15: a new install names no inverter. When that is the ONLY
+        # refusal it serves the setup wizard in setup-only mode, which builds
+        # nothing that commands and returns when stopped. Any other refusal
+        # still exits 78 (DECISIONS/UX-15-setup-only-mode.md).
+        if _problems and setuponly.applies(opts, _problems):
+            await setuponly.serve(
+                opts, HomeAssistant(session, read_only=True), store,
+                port=INGRESS_PORT)
+            return
         if _problems:
             refuse(_problems)
             raise ConfigRefused(_problems)
@@ -6054,7 +6070,7 @@ async def amain() -> None:
 
         stop = asyncio.Event()
         controller = Controller(opts, hass, store, pub, stop=stop)
-        runner = await web.start(controller, port=8099)
+        runner = await web.start(controller, port=INGRESS_PORT)
 
         loop = asyncio.get_running_loop()
         for sig in (signal.SIGTERM, signal.SIGINT):