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/css/chat.css

@@ -436,12 +436,22 @@
   color: var(--dangerous);
 }
 
-/* Plan Mode Toggle */
+/* Unified Modus-Toggle: Aus / Plan / Fusion / MoA (ein einziger #planModeBtn) */
 #planModeBtn.plan-mode-active {
   color: var(--primary);
   background: color-mix(in srgb, var(--primary) 15%, transparent);
   border-radius: var(--border-radius);
 }
+#planModeBtn.panel-mode-fusion {
+  color: var(--primary);
+  background: color-mix(in srgb, var(--primary) 15%, transparent);
+  border-radius: var(--border-radius);
+}
+#planModeBtn.panel-mode-moa {
+  color: var(--primary);
+  background: color-mix(in srgb, var(--primary) 28%, transparent);
+  border-radius: var(--border-radius);
+}
 
 /* Emoji Picker */