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/app.js

@@ -949,6 +949,7 @@
                         include_mid: memMid,
                         include_short: memShort,
                         off_record: current_mode === 'otr',
+                        model: primaryBackend,
                         user: CORTEX_USER,
                         persona: CORTEX_PERSONA,
                     }),
@@ -984,10 +985,15 @@
                             const assistHistIdx = currentHistory.length;
                             currentHistory.push({ role: 'assistant', content: data.response });
                             attachHistoryControls(thinkingDiv, assistHistIdx);
-                            if (data.fallback_used) {
-                                addMessage('system',
-                                    `⚡ ${primaryBackend} unavailable — answered by ${data.backend}`);
-                            }
+
+                            // Model tag — always shown, amber if fallback was used
+                            const modelTag = document.createElement('div');
+                            modelTag.className = 'model-tag' + (data.fallback_used ? ' fallback' : '');
+                            const label = data.backend_label || data.backend || '';
+                            modelTag.textContent = data.fallback_used
+                                ? `⚡ fallback → ${label}`
+                                : label;
+                            thinkingDiv.appendChild(modelTag);
                         } else if (data.type === 'error') {
                             throw new Error(data.message);
                         }