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/modals.js

@@ -192,6 +192,61 @@ function _openPickerModal(title, items, currentId, onSelect, options) {
   });
 }
 
+// ── Multi-Select Picker Modal ──
+// Wie _openPickerModal, aber Mehrfachauswahl: Karten togglen ihren selected-Zustand,
+// ein "Übernehmen"-Button bestätigt. onConfirm bekommt das Array der gewählten _pickerId.
+
+function _openMultiPickerModal(title, items, selectedIds, onConfirm, options) {
+  options = options || {};
+  var selected = new Set(selectedIds || []);
+
+  var overlay = document.createElement('div');
+  overlay.className = 'agent-picker-overlay';
+
+  var modal = document.createElement('div');
+  modal.className = 'agent-picker-modal';
+
+  var header = document.createElement('div');
+  header.className = 'agent-picker-header';
+  header.innerHTML = '<span>' + esc(title) + '</span><button class="icon-btn agent-picker-close"><i class="fa fa-times"></i></button>';
+  modal.appendChild(header);
+
+  var body = document.createElement('div');
+  body.className = 'agent-picker-body';
+
+  for (var i = 0; i < items.length; i++) {
+    (function(item) {
+      var cardEl = options.buildCard(item);
+      cardEl.classList.add('agent-picker-item');
+      if (selected.has(item._pickerId)) cardEl.classList.add('selected');
+      cardEl.addEventListener('click', function() {
+        if (selected.has(item._pickerId)) { selected.delete(item._pickerId); cardEl.classList.remove('selected'); }
+        else { selected.add(item._pickerId); cardEl.classList.add('selected'); }
+      });
+      body.appendChild(cardEl);
+    })(items[i]);
+  }
+  modal.appendChild(body);
+
+  var footer = document.createElement('div');
+  footer.style.cssText = 'display:flex;justify-content:flex-end;gap:8px;padding:12px;border-top:1px solid var(--f2-border)';
+  var confirmBtn = document.createElement('button');
+  confirmBtn.className = 'add-cmd-btn';
+  confirmBtn.textContent = 'Übernehmen';
+  confirmBtn.addEventListener('click', function() {
+    onConfirm(Array.from(selected));
+    overlay.remove();
+  });
+  footer.appendChild(confirmBtn);
+  modal.appendChild(footer);
+
+  overlay.appendChild(modal);
+  document.body.appendChild(overlay);
+
+  header.querySelector('.agent-picker-close').addEventListener('click', function() { overlay.remove(); });
+  overlay.addEventListener('click', function(e) { if (e.target === overlay) overlay.remove(); });
+}
+
 // ── Telegram Bot Picker Modal ──
 
 function openTelegramBotPickerModal(bots, currentBotId, onSelect) {