Files view: surface fetch errors instead of "no operations" (v2.3.1)

- `API._fetch` never checked the HTTP status: an error body (e.g. the 404
  `{"error": "Session not found"}`) was returned as data, so the Files view
  rendered it as "No file operations recorded" — and, worse, that bad payload
  was cached per session id, which is why only re-opening the session (which
  resets the cache) appeared to fix it.  It now throws with the server's
  message for any non-OK response (all 13 `await API.*` call sites are inside
  try/catch) and marks the server unavailable only for a real network error
- `renderFileView` accepts a payload only when it carries a `timeline` array:
  a genuine `timeline_total === 0` keeps the empty state, anything else shows
  "Failed to load files: <reason>"; failures are not cached, so the next visit
  retries; a response that arrives after the user switched sessions is ignored
- tests: HTTP-error vs network-error behaviour of `_fetch` (network error is
  the only disconnected state), the valid/empty/error render paths and the
  retry-after-error, the stale-response guard
- v2.3.1; SW cache simtrace-analyser-v64

frontend/index.html

@@ -1161,14 +1161,28 @@ const API = {
             opts.headers['Content-Type'] = 'application/json';
             opts.body = JSON.stringify(body);
         }
+        let res;
         try {
-            const res = await fetch(API_BASE() + path, opts);
-            return await res.json();
+            res = await fetch(API_BASE() + path, opts);
         } catch (e) {
+            // The server is unreachable (network error) — that *is* a
+            // disconnected state.
             STATE.serverAvailable = false;
             updateCaptureButton();
             throw e;
         }
+        let data = {};
+        try {
+            data = await res.json();
+        } catch (e) { /* non-JSON body */ }
+        if (!res.ok) {
+            // The server answers errors as {"error": …}; surface them instead
+            // of letting callers treat the error body as data.
+            throw Object.assign(
+                new Error((data && data.error) || `HTTP ${res.status}`),
+                { status: res.status, code: data && data.code });
+        }
+        return data;
     },
     status()        { return this._fetch('GET', '/api/status'); },
     listSessions()  { return this._fetch('GET', '/api/sessions'); },
@@ -2105,19 +2119,31 @@ async function renderFileView() {
         view.innerHTML = `<div class="empty-state" style="height:auto;padding:24px;">${t('Open a saved session from the Sessions tab.')}</div>`;
         return;
     }
-    if (STATE.filesData === null || STATE.filesSessionId !== STATE.sessionViewId) {
+    const sid = STATE.sessionViewId;
+    let data = STATE.filesData;
+    // A failure is never cached: only a well-formed payload is remembered, so
+    // the next visit retries instead of showing a stale error.
+    if (data === null || STATE.filesSessionId !== sid ||
+        !Array.isArray(data.timeline)) {
         view.innerHTML = `<div class="empty-state" style="height:auto;padding:24px;">${t('Loading…')}</div>`;
         try {
-            STATE.filesData = await API.getSessionFiles(STATE.sessionViewId);
-            STATE.filesSessionId = STATE.sessionViewId;
+            const payload = await API.getSessionFiles(sid);
+            if (STATE.sessionViewId !== sid) return;   // the view moved on
+            if (!payload || !Array.isArray(payload.timeline)) {
+                throw new Error((payload && payload.error) || t('Failed to load files'));
+            }
+            data = payload;
+            STATE.filesData = payload;
+            STATE.filesSessionId = sid;
         } catch (e) {
+            if (STATE.sessionViewId !== sid) return;
             STATE.filesData = null;
-            view.innerHTML = `<div class="empty-state" style="height:auto;padding:24px;">${t('Failed to load files')}</div>`;
+            STATE.filesSessionId = null;
+            view.innerHTML = `<div class="empty-state" style="height:auto;padding:24px;">${t('Failed to load files')}: ${escapeHtml((e && e.message) || '')}</div>`;
             return;
         }
     }
-    const data = STATE.filesData;
-    const timeline = (data && data.timeline) || [];
+    const timeline = data.timeline;
     if (!timeline.length) {
         view.innerHTML = `<div class="empty-state" style="height:auto;padding:24px;">${t('No file operations recorded')}</div>`;
         return;
@@ -3046,7 +3072,7 @@ function onFilterChange() {
 }
 
 // ──────────────────── Versioning ────────────────────
-const PWA_VERSION = '2.3.0';
+const PWA_VERSION = '2.3.1';
 const MIN_SERVER_VERSION = '2.3.0';
 
 function parseVersion(v) {