feat(whatsapp): QWEN primary AI backend, Hermes fallback, conversation history, vehicle persistence, demo prompts

- Add QWEN (qwen3.6) as primary AI backend with short system prompt
- Hermes remains as fallback with 45s timeout
- Increase QWEN timeout to 35s, max_tokens to 4000
- Add conversation history loading from whatsapp_messages (last 4 msgs)
- Persist detected vehicle in whatsapp_sessions table
- Add 'limpiar chat' / 'nuevo chat' / 'reset' commands to clear history
- Fix CSS conflict: rename whatsapp chat-panel classes to wa-chat-panel
- Fix JS ID conflicts with chat.js widget (waChatPanel, waChatMessages, etc.)
- Improve no-stock response: conversational with alternatives
- Split search_query by | for multi-part lookups
- Add DEMO_PROMPTS.md and DEMO_PROMPTS_V2.md

pos/static/css/whatsapp.css

@@ -217,14 +217,14 @@
 
     /* ─── Right panel: chat view ─────────────────────────────────────── */
 
-    .chat-panel {
+    .wa-chat-panel {
       flex: 1;
       display: flex;
       flex-direction: column;
       background: var(--color-bg-base);
     }
 
-    .chat-panel__header {
+    .wa-chat-panel__header {
       display: flex;
       align-items: center;
       justify-content: space-between;
@@ -234,18 +234,18 @@
       flex-shrink: 0;
     }
 
-    .chat-panel__phone {
+    .wa-chat-panel__phone {
       font-weight: var(--font-weight-semibold);
       font-size: var(--text-body);
       color: var(--color-text-primary);
     }
 
-    .chat-panel__actions {
+    .wa-chat-panel__actions {
       display: flex;
       gap: var(--space-2);
     }
 
-    .chat-panel__messages {
+    .wa-chat-panel__messages {
       flex: 1;
       overflow-y: auto;
       padding: var(--space-4);
@@ -500,7 +500,7 @@
 
     @media (max-width: 768px) {
       .conv-panel { width: 100%; min-width: 0; }
-      .chat-panel { display: none; }
+      .wa-chat-panel { display: none; width: 100%; }
       .messenger.has-active-chat .conv-panel { display: none; }
-      .messenger.has-active-chat .chat-panel { display: flex; }
+      .messenger.has-active-chat .wa-chat-panel { display: flex !important; width: 100%; }
     }