fix: settings page and help docs updated for model registry V2

settings.html:
- Remove Gemini API Key section (keys now managed in Model Registry)
- Rename "Local Models" → "Model Registry" with updated description
  covering all providers (Anthropic, Google, local hosts)
- Update button text: "Manage local models" → "Manage models"

settings.py: remove dead gemini_key template variable lookups

HELP.md:
- Fix navigation path: ☰ → Account → Model Registry → Manage models
- Restructure Model Registry section as ordered steps (1: providers/hosts,
  2: add models, 3: assign roles) so dependency order is clear
- Add explicit note that accounts/hosts must exist before adding models

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/routers/settings.py

@@ -42,20 +42,11 @@ def _settings_page(username: str, personas: list[str], success: str = "", error:
     html = (_STATIC / "settings.html").read_text()
     html = html.replace("{{ username }}", username)
 
-    # Connected Google account
+    # Connected Google account (OAuth sign-in)
     auth_data    = _read_auth(username)
     google_email = auth_data.get("google_email") or ""
     html = html.replace("{{ google_email }}", google_email)
 
-    # Gemini API key — show masked hint only, never the full key
-    gemini_key = auth_data.get("gemini_api_key") or ""
-    if gemini_key:
-        hint = f"Saved (…{gemini_key[-4:]})"
-    else:
-        hint = "Using server key"
-    html = html.replace("{{ gemini_key_hint }}", hint)
-    html = html.replace("{{ gemini_key_set }}", "true" if gemini_key else "false")
-
     persona_items = "\n".join(
         f'''<li>
           <a href="/{username}/{p}" class="persona-link">{p}</a>