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/messages.py

@@ -382,6 +382,16 @@ def banner_stopped() -> str:
     return "Stopped — the inverters are set to 0 W"
 
 
+def banner_setup_only() -> str:
+    """UX-15: setup-only mode, where the add-on serves the page and commands
+    nothing (app/setuponly.py)."""
+    return "Setup: the add-on controls nothing until setup is done"
+
+
+def row_setup_only() -> str:
+    return "No inverter is named yet, so finish Setup and restart the add-on."
+
+
 def banner_shadow(latched: bool) -> str:
     """SAFETY-10: first in the cascade, never displaced by another banner.
 
@@ -861,6 +871,7 @@ HINTS = {
     "control_stopped": "Control stopped",
     "settings_saved": "Settings saved, they apply after a restart",
     "restart_requested": "Restart requested from the settings page",
+    "setup_only": "In setup, nothing is sent to any inverter",
     "roster": "Inverter list read: {n} inverters",
     "maint_history": "Maintenance history did not move over cleanly",
     "stop_restored": "{unit}: an earlier safety stop is back, it stays at 0 W",
@@ -952,6 +963,8 @@ def status_samples() -> list:
         ("banner_peak_reserve", None, banner_peak_reserve()),
         ("banner_power_too_high", None, banner_power_too_high()),
         ("banner_shadow", None, banner_shadow(False)),
+        ("banner_setup_only", None, banner_setup_only()),
+        ("row_setup_only", AMBER, row_setup_only()),
         ("banner_shadow", None, banner_shadow(True)),
         ("row_controller_starting", GREEN, row_controller_starting()),
         ("row_controller_ran", GREEN, row_controller_ran(2)),