chore: replace Unicode/emoji icons with Lucide React

Adds lucide-react and replaces all HTML entity and emoji icons across
the UI with proper SVG icon components. Tree-shaken — only imported
icons are bundled.

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

ui/src/pages/Dashboard/Dashboard.tsx

@@ -1,5 +1,6 @@
 import { useState, useMemo, useCallback } from 'react'
 import { useParams, useNavigate } from 'react-router'
+import { ExternalLink, AlertTriangle } from 'lucide-react'
 import {
   DataTable,
   DetailPanel,
@@ -344,7 +345,7 @@ export default function Dashboard() {
             navigate(`/exchanges/${row.executionId}`)
           }}
         >
-          ↗
+          <ExternalLink size={14} />
         </button>
       ),
     }
@@ -418,7 +419,7 @@ export default function Dashboard() {
             expandedContent={(row: Row) =>
               row.errorMessage ? (
                 <div className={styles.inlineError}>
-                  <span className={styles.inlineErrorIcon}>{'\u26A0'}</span>
+                  <span className={styles.inlineErrorIcon}><AlertTriangle size={14} /></span>
                   <div>
                     <div className={styles.inlineErrorText}>{row.errorMessage}</div>
                     <div className={styles.inlineErrorHint}>Click to view full stack trace</div>