feat(canvas): Responsive full-screen layout and dark theme

Phase 2 of Canvas Professional Upgrade:
- Update AtomizerCanvas with Lucide icons (MessageCircle, Plug, X)
- Update CanvasView page with dark theme styling
- Replace emoji buttons with Lucide icons (ClipboardList, Download, Trash2)
- Update Setup canvas tab for full-width responsive layout
- Use calc(100vh-6rem) for proper canvas height

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

atomizer-dashboard/frontend/src/components/canvas/AtomizerCanvas.tsx

@@ -7,6 +7,7 @@ import ReactFlow, {
   ReactFlowInstance,
 } from 'reactflow';
 import 'reactflow/dist/style.css';
+import { MessageCircle, Plug, X } from 'lucide-react';
 
 import { nodeTypes } from './nodes';
 import { NodePalette } from './palette/NodePalette';
@@ -159,7 +160,7 @@ function CanvasFlow() {
             }`}
             title="Toggle Chat"
           >
-            {isConnected ? '💬' : '🔌'}
+            {isConnected ? <MessageCircle size={18} /> : <Plug size={18} />}
           </button>
           <button
             onClick={handleValidate}
@@ -206,7 +207,7 @@ function CanvasFlow() {
               onClick={() => setShowChat(false)}
               className="text-dark-400 hover:text-white transition-colors"
             >
-              ✕
+              <X size={18} />
             </button>
           </div>
           <ChatPanel