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

Endpoints/SessionEndpoint.cs

@@ -16,6 +16,7 @@ public static class SessionEndpoint
         app.MapGet("/api/sessions/{sessionId}", HandleGet);
         app.MapPost("/api/sessions", HandleCreate);
         app.MapPut("/api/sessions/{sessionId}", HandleUpdate);
+        app.MapDelete("/api/sessions/{sessionId}", HandleDelete);
     }
 
     private static async Task HandleList(HttpContext ctx, DatabaseService db)
@@ -43,7 +44,14 @@ public static class SessionEndpoint
 
     private static async Task HandleCreate(HttpContext ctx, DatabaseService db)
     {
-        var session = await db.CreateSession("Neue Session");
+        var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
+
+        var name = body.TryGetProperty("name", out var n) ? n.GetString() : "Neue Session";
+        var description = body.TryGetProperty("description", out var d) ? d.GetString() : null;
+        var image = body.TryGetProperty("image", out var img) ? img.GetString() : null;
+        var agentId = body.TryGetProperty("agentId", out var a) ? a.GetString() : null;
+
+        var session = await db.CreateSession(name, description, image, agentId);
         await ctx.Response.WriteAsJsonAsync(session);
     }
 
@@ -57,16 +65,18 @@ public static class SessionEndpoint
         }
 
         var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
-        if (body.TryGetProperty("name", out var nameEl))
-        {
-            await db.UpdateSessionName(sessionId, nameEl.GetString());
-        }
-        if (body.TryGetProperty("agentId", out var agentEl))
-        {
-            await db.UpdateSessionAgent(sessionId, agentEl.GetString());
-        }
+        await db.UpdateSession(sessionId, body);
 
         var session = await db.GetSession(sessionId);
         await ctx.Response.WriteAsJsonAsync(session);
     }
+
+    private static async Task HandleDelete(HttpContext ctx, DatabaseService db)
+    {
+        var sessionId = ctx.Request.RouteValues["sessionId"]?.ToString();
+        if (string.IsNullOrEmpty(sessionId)) { ctx.Response.StatusCode = 400; return; }
+
+        await db.DeleteSession(sessionId);
+        ctx.Response.StatusCode = 204;
+    }
 }