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/static/settings.html

@@ -269,44 +269,18 @@
       </p>
     </div>
 
-    <!-- Gemini API key -->
+    <!-- Model Registry link -->
     <div class="section">
-      <h2>Gemini API Key</h2>
+      <h2>Model Registry</h2>
       <p style="font-size:0.8rem; color:#94a3b8; margin-bottom:0.85rem; line-height:1.55;">
-        Paste your personal key from
-        <a href="https://aistudio.google.com/apikey" target="_blank" rel="noopener"
-           style="color:#a78bfa;">aistudio.google.com/apikey</a>
-        to use your own Gemini quota. Leave blank to use the shared server key.
-      </p>
-      <form method="POST" action="/settings/gemini-key">
-        <div class="field">
-          <label for="gemini_api_key">API Key</label>
-          <input type="text" id="gemini_api_key" name="gemini_api_key"
-                 placeholder="{{ gemini_key_hint }}"
-                 autocomplete="new-password" spellcheck="false"
-                 data-1p-ignore data-lpignore="true" data-form-type="other">
-        </div>
-        <button type="submit">Save Key</button>
-      </form>
-      <p id="gemini-key-status" style="font-size:0.75rem; color:#94a3b8; margin-top:0.5rem;">
-        Current: {{ gemini_key_hint }}
-        <span id="gemini-remove-wrap" style="{{ gemini_key_set == 'false' and 'display:none' or '' }}">
-          — <a href="#" id="gemini-remove-link" style="color:#f87171;">remove</a>
-        </span>
-      </p>
-    </div>
-
-    <!-- Local models link -->
-    <div class="section">
-      <h2>Local Models</h2>
-      <p style="font-size:0.8rem; color:#94a3b8; margin-bottom:0.85rem; line-height:1.55;">
-        Configure OpenAI-compatible hosts and models (Open WebUI, Ollama, LM Studio, etc.).
+        Configure AI providers (Anthropic, Google), local hosts (Open WebUI, Ollama, OpenRouter, etc.),
+        and assign models to roles — chat, orchestrator, distill, and more.
       </p>
       <a href="/settings/models"
          style="display:inline-block; padding:0.55rem 1rem; background:#7c3aed; border-radius:6px;
                 color:#fff; font-size:0.88rem; font-weight:600; text-decoration:none;
                 transition:background 0.15s;">
-        Manage local models →
+        Manage models →
       </a>
     </div>