feat: migrate all icons to Lucide React

Replace unicode characters, emoji, and inline SVGs with lucide-react
components across the entire design system and page layer. Update
tests to assert on SVG elements instead of text content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/composites/Toast/Toast.tsx

@@ -8,6 +8,7 @@ import {
   type ReactNode,
 } from 'react'
 import { createPortal } from 'react-dom'
+import { Info, CheckCircle, AlertTriangle, XCircle, X } from 'lucide-react'
 import styles from './Toast.module.css'
 
 // ── Types ──────────────────────────────────────────────────────────────────
@@ -39,11 +40,11 @@ const MAX_TOASTS = 5
 const DEFAULT_DURATION = 5000
 const EXIT_ANIMATION_MS = 300
 
-const ICONS: Record<ToastVariant, string> = {
-  info: 'ℹ',
-  success: '✓',
-  warning: '⚠',
-  error: '✕',
+const ICONS: Record<ToastVariant, ReactNode> = {
+  info: <Info size={16} />,
+  success: <CheckCircle size={16} />,
+  warning: <AlertTriangle size={16} />,
+  error: <XCircle size={16} />,
 }
 
 // ── Context ────────────────────────────────────────────────────────────────
@@ -183,7 +184,7 @@ function ToastItemComponent({ toast, onDismiss }: ToastItemComponentProps) {
         aria-label="Dismiss notification"
         type="button"
       >
-        &times;
+        <X size={14} />
       </button>
     </div>
   )