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

wwwroot/css/form.css

@@ -10,6 +10,11 @@
   margin-top: var(--box-spacing);
 }
 
+.more {
+  margin: var(--box-spacing) auto;
+  cursor: pointer;
+}
+
 .avatar-section {
   display: flex;
   align-items: center;
@@ -55,7 +60,7 @@
 .form-actions {
   display: flex;
   gap: var(--badge-spacing);
-  margin-block: var(--box-spacing) calc(var(--page-spacing) * 4);
+  margin-block: var(--box-spacing) calc(var(--page-spacing) * 2);
 }
 
 /* ── Custom Commands ── */
@@ -211,12 +216,6 @@ label.skill-item {
   color: var(--dangerous);
 }
 
-.profile-divider {
-  border: none;
-  border-top: var(--border-width) solid var(--f2-border);
-  margin: var(--box-spacing) 0;
-}
-
 .danger-btn {
   color: var(--dangerous);
   border-color: currentcolor;