feat: persist active session across page navigation with inactivity TTL

Session ID is stored in localStorage keyed to user+persona. On page load
it's silently restored if within 30 min of last activity. Timestamp
updates on every sent message. New session / delete session clears the
stored ID so the TTL logic stays consistent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/app.js

@@ -42,6 +42,31 @@
         let currentHistory   = [];  // mirrors backend session [{role, content}, ...]
         let talkThinkingDiv  = null; // pending "thinking…" bubble for live Talk updates
 
+        // ── Session persistence ───────────────────────────────────────
+        // Survives page navigation (help, settings, etc.) within the same browser.
+        // Expires after SESSION_TTL_MS of inactivity.
+        const SESSION_TTL_MS  = 30 * 60 * 1000;  // 30 minutes
+        const _sid_key        = `cx_sid_${CORTEX_USER}_${CORTEX_PERSONA}`;
+        const _sid_ts_key     = `cx_sid_ts_${CORTEX_USER}_${CORTEX_PERSONA}`;
+
+        function persist_session() {
+            if (!sessionId) return;
+            localStorage.setItem(_sid_key, sessionId);
+            localStorage.setItem(_sid_ts_key, String(Date.now()));
+        }
+
+        function clear_stored_session() {
+            localStorage.removeItem(_sid_key);
+            localStorage.removeItem(_sid_ts_key);
+        }
+
+        function get_stored_session() {
+            const id = localStorage.getItem(_sid_key);
+            const ts = parseInt(localStorage.getItem(_sid_ts_key) || '0', 10);
+            if (!id || Date.now() - ts > SESSION_TTL_MS) return null;
+            return id;
+        }
+
         // ── Enter toggle ─────────────────────────────────────────────
         // Default: Ctrl+Enter sends. Stored in localStorage.
         let ctrlEnterMode = localStorage.getItem('ctrlEnterSend') !== 'false';
@@ -314,6 +339,7 @@
             const newItem = makeItem('new', '+ New session', '');
             newItem.addEventListener('click', () => {
                 sessionId = null;
+                clear_stored_session();
                 currentHistory = [];
                 messagesEl.innerHTML = '';
                 sessionEl.textContent = '';
@@ -392,6 +418,7 @@
                     await fetch(`/sessions/${s.session_id}?${_fileParams}`, { method: 'DELETE' });
                     if (sessionId === s.session_id) {
                         sessionId = null;
+                        clear_stored_session();
                         currentHistory = [];
                         messagesEl.innerHTML = '';
                         sessionEl.textContent = '';
@@ -425,10 +452,11 @@
             return item;
         }
 
-        async function resumeSession(id) {
+        async function resumeSession(id, silent = false) {
             talkThinkingDiv = null;
             if (id && id.startsWith('nct_')) sessionsBtn.classList.remove('talk-badge');
             const res = await fetch(`/history/${id}?${_fileParams}`);
+            if (!res.ok) throw new Error(`HTTP ${res.status}`);
             const data = await res.json();
 
             messagesEl.innerHTML = '';
@@ -444,10 +472,11 @@
                 attachHistoryControls(msgDiv, i);
             }
 
-            addMessage('system', `Resumed session ${id}`);
+            if (!silent) addMessage('system', `Resumed session ${id}`);
             scrollToBottom();
             sessionsPanel.classList.remove('open');
             inputEl.focus();
+            persist_session();
         }
 
         function timeAgo(iso) {
@@ -793,6 +822,7 @@
                         if (data.type === 'response') {
                             sessionId = data.session_id;
                             sessionEl.textContent = `session: ${sessionId}`;
+                            persist_session();
                             thinkingDiv.className = 'message assistant';
                             setMessageText(thinkingDiv, 'assistant', data.response);
                             const assistHistIdx = currentHistory.length;
@@ -893,6 +923,7 @@
                 if (job.session_id) {
                     sessionId = job.session_id;
                     sessionEl.textContent = `session: ${sessionId}`;
+                    persist_session();
                 }
 
                 const userHistIdx = currentHistory.length - 1; // pushed before fetch
@@ -1315,3 +1346,10 @@
         // and seed the mode UI (which also calls render_icons internally).
         update_mode_ui();
         render_icons();
+
+        // ── Auto-restore last session ─────────────────────────────────
+        // Silently resume if within the inactivity TTL; clears stored ID on error.
+        {
+            const stored = get_stored_session();
+            if (stored) resumeSession(stored, true).catch(clear_stored_session);
+        }