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/hass.py

@@ -51,10 +51,18 @@ class Outage:
 OUTAGE = Outage()
 
 
+class ReadOnly(RuntimeError):
+    """A service call on a read-only HomeAssistant (UX-15 setup-only mode)."""
+
+
 class HomeAssistant:
-    def __init__(self, session: aiohttp.ClientSession, token: str | None = None):
+    def __init__(self, session: aiohttp.ClientSession, token: str | None = None,
+                 *, read_only: bool = False):
         self.session = session
         self.token = token or os.environ.get("SUPERVISOR_TOKEN", "")
+        # UX-15: setup-only mode reads Home Assistant and calls no service.
+        # Fixed at construction; `call()` refuses before it sends anything.
+        self._read_only = bool(read_only)
         self._last_moan: dict[str, float] = {}
         if not self.token:
             _LOG.error("SUPERVISOR_TOKEN missing - is this running as an add-on?")
@@ -181,7 +189,17 @@ class HomeAssistant:
                 return None
         return (_f("min"), _f("max"), _f("step"))
 
+    @property
+    def read_only(self) -> bool:
+        return self._read_only
+
     async def call(self, domain: str, service: str, data: dict) -> bool:
+        if self._read_only:
+            # Every service call, `number.set_value` included, goes through
+            # here, so setup-only mode has no way to command (DECISIONS UX-15).
+            _LOG.error("Setup-only mode called no service, because it may not "
+                       "command anything.")
+            raise ReadOnly(f"{domain}.{service} refused: setup-only mode")
         try:
             async with self.session.post(
                 f"{CORE_API}/services/{domain}/{service}",