feat: shared-page message merging + UI reorganization

Shared Page:
- Merge consecutive assistant text messages (same iteration)
- Add iteration field to thread API response
- Unified rendering for single/thread messages

UI Reorganization (manual changes):
- Session + Agent forms: collapsible 'Weitere Einstellungen' section
- Agent defaults: tool_choice unchecked, web_search 2025, model opus-4-6
- Profile: default-path field hidden, divider removed
- Form spacing adjustments, .more toggle style

Endpoints/SharedEndpoint.cs

@@ -126,7 +126,8 @@ public static class SharedEndpoint
                     m.Id,
                     m.Role,
                     m.Content,
-                    m.CreatedAt
+                    m.CreatedAt,
+                    m.Iteration
                 })
                 .ToList();