fix: add logout button to vendor onboarding page

The onboarding page was a standalone page with no way to log out,
leaving users stuck. Added:
- Logout button in the header
- handleLogout() function to clear vendor tokens and redirect to login

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/vendor/onboarding.html

@@ -23,6 +23,12 @@
                     </div>
                     <span class="text-xl font-semibold text-gray-800 dark:text-white">Wizamart</span>
                 </div>
+                <!-- Logout Button -->
+                <button @click="handleLogout()"
+                        class="mr-4 px-3 py-2 text-sm font-medium text-gray-600 dark:text-gray-300 hover:text-gray-900 dark:hover:text-white hover:bg-gray-100 dark:hover:bg-gray-700 rounded-lg transition-colors">
+                    Logout
+                </button>
+
                 <!-- Language Selector -->
                 <div class="relative" x-data="{ open: false }">
                     <button @click="open = !open"