fix: backend toggle not sent to server; add per-message model tag

Fixes:
  - app.js was tracking primaryBackend locally but never included
    model: primaryBackend in the /chat POST body, so the server always
    used settings.primary_backend regardless of what the user clicked.
    Now model: primaryBackend is sent on every chat request.

  - Responses were only annotated when fallback occurred. Now every
    assistant message shows a small model tag at the bottom right.

chat.py:
  - _backend_label() resolves human-readable name:
      claude → "Claude", gemini → "Gemini",
      local → registry label (e.g. "Gemma 4 E4B") or model_name
  - SSE payload now includes backend_label field

app.js:
  - model: primaryBackend added to /chat fetch body
  - After every response, appends .model-tag div with backend_label
  - Fallback shows "⚡ fallback → <label>" in amber; normal is muted
  - Removed separate system message for fallback (tag covers it)

style.css:
  - .model-tag: small muted text, right-aligned, separated by thin line
  - .model-tag.fallback: amber (#f59e0b)

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

cortex/static/style.css

@@ -552,6 +552,20 @@
         .copy-btn:hover  { color: var(--text); border-color: var(--muted); }
         .copy-btn.copied { color: var(--success); border-color: var(--success-dim); }
 
+        /* Model tag — shown at the bottom of every assistant message */
+        .model-tag {
+            display: block;
+            font-size: 0.67rem;
+            color: #334155;
+            margin-top: 0.55rem;
+            padding-top: 0.4rem;
+            border-top: 1px solid #1e2030;
+            text-align: right;
+            letter-spacing: 0.02em;
+            font-weight: 500;
+        }
+        .model-tag.fallback { color: #f59e0b; }
+
         /* Note messages */
         .message.note-private {
             align-self: flex-end;