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/DOCS.md

@@ -3233,19 +3233,35 @@ the last step.
   sign for you; with a net or import/export meter it tells you what to change.
 - **The last step** runs the add-on's own start checks and reads Home
   Assistant: the meter reading, that each power request entity exists, the
-  battery levels, and whether prices are known. A missing power request entity
-  is red, so it cannot be saved.
+  battery levels, and whether prices are known. A missing power request or
+  meter entity is red, so it cannot be saved.
 - **Save and restart** saves, then restarts the add-on. Every inverter is set
   to 0 W while it restarts, and with "Start in shadow mode" on (the default) it
   stays at 0 W until you turn shadow mode off under Settings.
 
 Setup opens by itself on a new install's first start, until Setup finishes or
 control is started, and whenever the stored settings name no inverter to
-control. A site that already ran before is never sent there. The add-on does
-not start with no power request entity at all, so on a brand-new install fill
-in "Inverter setpoint entity" on the Configuration tab first.
+control. A site that already ran before is never sent there.
 
-*For installers: card UX-13, and `DECISIONS/UX-13-setup-wizard.md`.*
+A new install names no inverter yet, so the add-on starts **in setup**: it
+serves the page and sends nothing to any inverter. Home says "Setup: the
+add-on controls nothing until setup is done", and there is no Start button. The
+installer's steps:
+
+1. Install the add-on, turn on **Watchdog** on its Info tab, and start it.
+   Leave the Configuration tab as it is.
+2. Open the add-on's page. Setup opens by itself.
+3. Go through the six steps and press **Save and restart**. With "Start in
+   shadow mode" on, every inverter stays at 0 W after the restart.
+4. On Home, check the meter direction and what the add-on would do. Turn
+   shadow mode off under Settings when it looks right.
+
+Any other problem in the stored settings still stops the add-on (exit code 78),
+and the log names each one. Fix those on the Configuration tab.
+
+*For installers: cards UX-13 and UX-15, `DECISIONS/UX-13-setup-wizard.md` and
+`DECISIONS/UX-15-setup-only-mode.md`. In setup the `status` JSON has
+`setup_only: true`, and no MQTT entity is published.*
 
 ## Status page
 
@@ -3260,6 +3276,10 @@ inverter stays at 0 W. If the banner is red and says a safety stop stays on,
 one of the red rows below is a stop that will still be there when shadow mode
 ends. That row says how to clear it.
 
+In **setup** (a new install that names no inverter yet) the banner says the
+add-on controls nothing until setup is done, and the one row says to finish
+Setup. See *Setup, for a new install*.
+
 | Row starts with | What it tells you |
 |---|---|
 | Control | When control last ran. Red when it has stopped running: the control board then sets the inverter to 0 W. |