Make remember checkboxes more subtle and checked by default

- Remember checkboxes now default to checked for better user experience
- Reduced visual prominence with smaller size, lighter colors, and lower opacity
- Users get convenience by default while still being able to opt out

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

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

frontend/styles.css

@@ -292,22 +292,25 @@ body {
 .remember-checkbox {
     display: flex;
     align-items: center;
-    gap: 0.5rem;
-    margin-top: 0.5rem;
+    gap: 0.375rem;
+    margin-top: 0.375rem;
+    opacity: 0.7;
 }
 
 .remember-checkbox input[type="checkbox"] {
     width: auto;
     margin: 0;
     cursor: pointer;
+    transform: scale(0.85);
 }
 
 .remember-checkbox label {
     margin: 0;
-    font-size: 0.875rem;
-    color: #666;
+    font-size: 0.75rem;
+    color: #888;
     cursor: pointer;
     user-select: none;
+    font-weight: 400;
 }
 
 .login-button, .register-button {