fix: admin logout clears only admin tokens + add signup token tests

- Fix admin header.html logout to not use localStorage.clear()
  which was clearing vendor/customer tokens too
- Add tests for signup access_token generation
- Test that token is returned in response
- Test that token can authenticate API calls
- Test that vendor_token cookie is set

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

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

app/templates/admin/partials/header.html

@@ -284,8 +284,10 @@ document.addEventListener('alpine:init', () => {
                     console.error('Logout API error (continuing anyway):', error);
                 })
                 .finally(() => {
-                    // Clear all tokens
-                    localStorage.clear();
+                    // Clear admin tokens only (not vendor or customer tokens)
+                    localStorage.removeItem('admin_token');
+                    localStorage.removeItem('admin_user');
+                    // Note: Do NOT use localStorage.clear() - it would clear vendor/customer tokens too
                     window.location.href = '/admin/login';
                 });
             }