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/store.py

@@ -581,9 +581,12 @@ def migrate_maint_keys(store, unit: str, roster) -> MigrationReport:
     return rep
 
 
+STATE_PATH = "/data/state.json"
+
+
 class Store:
-    def __init__(self, path: str = "/data/state.json"):
-        self.path = path
+    def __init__(self, path: str | None = None):
+        self.path = path or STATE_PATH
         self.data = dict(DEFAULTS)
         # UX-13 P4: True when this boot found no state file, i.e. a new /data.
         # Only the setup wizard's first-run rule reads it (uipage.setup_needed).