Merge plan and panel toggles into one composer button

The composer had two separate mode buttons (plan + panel). Fold them into a
single #planModeBtn that cycles through every operating mode in one place:
Off -> Plan -> Fusion -> MoA -> Off. Cleaner UI, one affordance instead of two.

Plan stays transient (reset on session switch, never persisted) while
Fusion/MoA persist to the session via apiUpdateSession({multiModelMode}) and
are read server-side from ctx.MultiModelMode. The compat var _planModeActive
is kept in sync (true iff mode === 'plan') so sendMessage and modals.js read
it unchanged. No text label — icon + tint only.

- index.html: drop the separate panel button, reuse #planModeBtn
- chat.css: unified states (plan/fusion/moa tints) on #planModeBtn
- chat.js: cycleCompositeMode / initCompositeMode / _renderCompositeBtn
  replace the split plan/panel functions
- chat-page.js: single wiring block, init from session.multiModelMode
- ARCHITECTURE_QUICK.md: document the unified toggle

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

wwwroot/js/pages/chat-page.js

@@ -572,13 +572,12 @@ registerPage('chat', {
       });
     }
 
-    // Plan Mode Toggle
-    const planModeBtn = document.getElementById('planModeBtn');
-    if (planModeBtn) {
-      // Plan-Modus bei Session-Wechsel zurücksetzen
-      _planModeActive = false;
-      planModeBtn.classList.remove('plan-mode-active');
-      planModeBtn.addEventListener('click', togglePlanMode);
+    // Unified Modus-Toggle (Aus / Plan / Fusion / MoA) – aus der Session initialisieren
+    const compositeModeBtn = document.getElementById('planModeBtn');
+    if (compositeModeBtn) {
+      const hasPanelAgents = !!(session.panelAgentIds && String(session.panelAgentIds).trim());
+      initCompositeMode(session.multiModelMode, hasPanelAgents);
+      compositeModeBtn.addEventListener('click', cycleCompositeMode);
     }
 
     // File Upload