fix: auto-login after signup and context-aware token clearing

This fixes the "Authorization header required for API calls" error during
vendor onboarding after signup.

Changes:
- Generate JWT access token on signup completion
- Set vendor_token cookie for page navigation
- Return access_token in signup response for localStorage
- Store vendor_token in localStorage after signup completion
- Make clearTokens() context-aware to prevent cross-portal interference
- Fix vendor logout to not clear admin/customer tokens

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

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

app/templates/platform/signup.html

@@ -511,6 +511,12 @@ function signupWizard() {
 
                 const data = await response.json();
                 if (response.ok) {
+                    // Store access token for automatic login
+                    if (data.access_token) {
+                        localStorage.setItem('vendor_token', data.access_token);
+                        localStorage.setItem('vendorCode', data.vendor_code);
+                        console.log('Vendor token stored for automatic login');
+                    }
                     window.location.href = '/signup/success?vendor_code=' + data.vendor_code;
                 } else {
                     alert(data.detail || 'Failed to complete signup');