Sessions: edit form, soft-delete, lazy creation, chat as separate page

- Session click opens edit form (name, description, image, agent) instead of chat
- Chat is now a separate route (#chat/{id}), opened via button
- Sessions created on save, not on click
- Soft-delete for sessions (is_deleted flag)
- Delete button in session form
- Removed title double-click editing from chat
- Card images now square (aspect-ratio: 1)
- New Session as placeholder card

wwwroot/js/api.js

@@ -76,8 +76,12 @@ async function apiGetSessions() {
   return resp.json();
 }
 
-async function apiCreateSession() {
-  const resp = await fetch('/api/sessions', { method: 'POST' });
+async function apiCreateSession(data = {}) {
+  const resp = await fetch('/api/sessions', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(data)
+  });
   return resp.json();
 }
 
@@ -87,6 +91,10 @@ async function apiGetSession(sessionId) {
   return resp.json();
 }
 
+async function apiDeleteSession(sessionId) {
+  return fetch(`/api/sessions/${sessionId}`, { method: 'DELETE' });
+}
+
 async function apiUpdateSession(sessionId, data) {
   const resp = await fetch(`/api/sessions/${sessionId}`, {
     method: 'PUT',