feat(snippets): add snippets entity, crud, and auto-browser-per-run

- add Snippet entity with name, description, code; full CRUD backend
- add snippets pages (list, create, edit, detail) and nav entry
- add runSnippet helper in code-executor using new Function with args array
- add result param to execute() so validateCode can access exec output
- remove sessionName from steps; each run now spawns its own fresh browser
- fix waitForURL race by polling localStorage for token instead

client/src/pages/scenario/EditStepPage.tsx

@@ -18,10 +18,8 @@ export function EditStepPage() {
   const [order, setOrder] = useState('');
   const [title, setTitle] = useState('');
   const [type, setType] = useState<StepType>('exec');
-  const [sessionName, setSessionName] = useState('');
   const [execCode, setExecCode] = useState('');
   const [validateCode, setValidateCode] = useState('');
-  const [sessionError, setSessionError] = useState('');
   const [orderError, setOrderError] = useState('');
   const [loading, setLoading] = useState(true);
   const [saving, setSaving] = useState(false);
@@ -36,7 +34,6 @@ export function EditStepPage() {
         setOrder(String(st.order));
         setTitle(st.title ?? '');
         setType(st.type);
-        setSessionName(st.sessionName);
         setExecCode(st.execCode ?? '');
         setValidateCode(st.validateCode ?? '');
       })
@@ -46,10 +43,6 @@ export function EditStepPage() {
 
   const validate = (): boolean => {
     let valid = true;
-    if (!sessionName.trim()) {
-      setSessionError(t('steps.form_session_required'));
-      valid = false;
-    }
     if (isNaN(Number(order)) || Number(order) < 0) {
       setOrderError(t('steps.form_order_invalid'));
       valid = false;
@@ -67,7 +60,6 @@ export function EditStepPage() {
         order: Number(order),
         title: title.trim() || undefined,
         type,
-        sessionName: sessionName.trim(),
         execCode: execCode.trim() || undefined,
         validateCode: validateCode.trim() || undefined,
       });
@@ -124,17 +116,6 @@ export function EditStepPage() {
                 onChange={(e) => setType(e.target.value as StepType)}
                 options={STEP_TYPES.map((v) => ({ value: v, label: v }))}
               />
-              <Input
-                label={t('steps.form_session')}
-                placeholder={t('steps.form_session_placeholder')}
-                value={sessionName}
-                onChange={(e) => {
-                  setSessionName(e.target.value);
-                  setSessionError('');
-                }}
-                error={sessionError || undefined}
-                required
-              />
               <div className={styles.formField}>
                 <label className={styles.fieldLabel}>{t('steps.form_exec_code')}</label>
                 <textarea