feat: add 'Keep me logged in' checkbox to login form

- index.html: checkbox between password field and login button
- app.js: reads #remember-me and passes rememberMe in POST body
- auth.js: rememberMe=true sets 30-day maxAge; false = session cookie
  (expires when browser closes)
- style.css: .form-group--checkbox and .checkbox-label styles

public/style.css

@@ -612,6 +612,32 @@ body {
   border-color: var(--accent);
 }
 
+.form-group--checkbox {
+  margin-bottom: 20px;
+}
+
+.checkbox-label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  cursor: pointer;
+  font-size: 0.9rem;
+  color: var(--text-secondary);
+  user-select: none;
+}
+
+.checkbox-label input[type="checkbox"] {
+  width: 16px;
+  height: 16px;
+  accent-color: var(--accent);
+  cursor: pointer;
+  flex-shrink: 0;
+}
+
+.checkbox-label span {
+  line-height: 1;
+}
+
 .login-btn {
   width: 100%;
   padding: 10px;