feat(pos): add inventory-aware chat context (#31) and VIN decoder (#17)

Chat now fetches tenant inventory summary (brands, counts, low-stock)
and injects it into the AI system prompt so responses prioritize local
stock. VIN decoder uses free NHTSA vPIC API to decode 17-char VINs and
auto-fills the vehicle selector dropdowns when a catalog match is found.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/templates/catalog.html

@@ -634,6 +634,17 @@
             </select>
           </div>
           <button class="vs-clear" id="vsClear" onclick="CatalogApp.vsClear()" title="Limpiar seleccion" style="display:none;">✕</button>
+          <span class="vs-vin-divider" style="color:var(--color-text-disabled);padding-bottom:6px;flex-shrink:0;">|</span>
+          <div class="vs-group" id="vinGroup" style="position:relative;">
+            <a class="vs-label" id="vinToggle" href="#" onclick="event.preventDefault();CatalogApp.toggleVin();" style="color:var(--color-primary);cursor:pointer;text-decoration:underline;white-space:nowrap;">Tienes el VIN?</a>
+            <div id="vinInputWrap" style="display:none;">
+              <div style="display:flex;gap:4px;">
+                <input type="text" class="vs-select" id="vinInput" placeholder="Ej: 1HGBH41JXMN109186" maxlength="17" style="text-transform:uppercase;font-family:var(--font-mono,monospace);letter-spacing:0.05em;flex:1;" />
+                <button class="btn btn-primary" id="vinDecodeBtn" onclick="CatalogApp.decodeVin()" style="height:auto;padding:var(--space-2) var(--space-3);font-size:var(--text-body-sm);">Decodificar</button>
+              </div>
+              <div id="vinStatus" style="font-size:var(--text-caption);margin-top:4px;color:var(--color-text-muted);display:none;"></div>
+            </div>
+          </div>
         </div>
       </div>