feat: session delete + touch-friendly message controls

Session delete:
- DELETE /sessions/{session_id} endpoint (chat.py + session_store.py)
- × button on each session item in the panel (hover-reveal on desktop)
- Clears UI if the active session is deleted

Touch accessibility:
- @media (hover: none) rule makes msg-actions always visible on touch devices
- msg-act-btn tap targets enlarged to 36px min-height, readable font size
- session-delete-btn also always visible and finger-sized on touch

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

cortex/static/app.js

@@ -183,6 +183,27 @@
                     `${s.message_count} msgs · ${timeAgo(s.updated)}`
                 );
                 item.addEventListener('click', () => resumeSession(s.session_id));
+
+                const delBtn = document.createElement('button');
+                delBtn.className = 'session-delete-btn';
+                delBtn.textContent = '×';
+                delBtn.title = 'Delete session';
+                delBtn.addEventListener('click', async (e) => {
+                    e.stopPropagation();
+                    await fetch(`/sessions/${s.session_id}`, { method: 'DELETE' });
+                    if (sessionId === s.session_id) {
+                        sessionId = null;
+                        currentHistory = [];
+                        messagesEl.innerHTML = '';
+                        sessionEl.textContent = '';
+                        addMessage('system', 'Session deleted');
+                    }
+                    const res = await fetch('/sessions');
+                    const data = await res.json();
+                    renderPanel(data.sessions);
+                });
+                item.appendChild(delBtn);
+
                 sessionsPanel.appendChild(item);
             }
         }