Add multi-model panel with Fusion mode

Panel agents answer the user prompt in parallel (read-only, non-streaming);
their responses are merged into the worker's last user message as a
<panel_responses> block, and the session agent synthesizes one final
streamed answer. The RunPanel engine is built layer-ready for future MoA.

- New session fields: multi_model_mode (off/fusion), panel_agent_ids (CSV)
  with idempotent column migration
- New partial Services/AnthropicService.Panel.cs holding the RunPanel engine
- Provider-agnostic members: Anthropic RunNonStreamingToolLoop / OpenAI
  CompleteText / CLI plan-mode; all forced read-only, persisted with Tag=panel
- SSE events: panel_start, panel_member_start, panel_member_done, panel_done
- Frontend: multi-agent picker, live panel bubbles, history rendering
- Graceful degradation: panel errors fall back to worker-only

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/js/stream-ui.js

@@ -205,3 +205,42 @@ function finalizeReviewBubble(type, data, agentType) {
   _currentReviewToolBox = null;
   scrollDown();
 }
+
+// ── Panel Bubbles (Fusion: mehrere Member parallel, non-streaming) ──
+// Member liefern ihren Text gesammelt bei panel_member_done → pro Member eine eigene
+// Bubble (keyed by index), kein Konflikt mit dem Single-_reviewBubble-State.
+
+let _panelBubbles = {};
+
+function startPanel(data) {
+  endStreamBubble();
+  _panelBubbles = {};
+}
+
+function addPanelMemberBubble(data) {
+  var bubble = document.createElement('div');
+  bubble.className = 'msg review-bubble panel-bubble streamed';
+  var model = (typeof prettyModelName === 'function') ? prettyModelName(data.model) : data.model;
+  bubble.appendChild(_createSubAgentHeader('panel', data.name, { model: model }));
+  var content = document.createElement('div');
+  content.className = 'review-content markdown';
+  content.innerHTML = '<span class="tool-spinner"><i class="fa fa-hourglass-half"></i></span> denkt nach…';
+  bubble.appendChild(content);
+  bubble._panelContent = content;
+  _panelBubbles[data.index] = bubble;
+  chatEl().appendChild(bubble);
+  scrollDown();
+}
+
+function finalizePanelMember(data) {
+  var bubble = _panelBubbles[data.index];
+  if (!bubble) return;
+  var text = (data.text && data.text.trim()) ? data.text : '_(keine Antwort)_';
+  bubble._panelContent.innerHTML = renderMarkdown(text);
+  delete _panelBubbles[data.index];
+  scrollDown();
+}
+
+function endPanel(data) {
+  _panelBubbles = {};
+}