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/static/index.html

@@ -72,6 +72,7 @@
     <div class="small" id="ctl-text"></div>
     <div class="row" style="margin-top:10px">
       <button class="btn" id="btn-ctl" type="button"></button>
+      <a class="btn primary" id="btn-setup" href="#setup" hidden data-t="setup_only.open"></a>
       <a class="small" id="docs-link" hidden target="_top" rel="noopener" data-t="control.docs"></a>
     </div>
     <div class="tiny muted" id="hint" aria-live="polite" style="margin-top:8px"></div>