feat: match filter input to sidebar search styling

Add search icon, translucent background, and same padding/sizing
as the sidebar's built-in filter input. Placeholder changed to
"Filter..." to match sidebar convention.

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

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -1,6 +1,6 @@
 import { useState, useMemo, useCallback, useRef, useEffect } from 'react';
 import { useParams, useNavigate } from 'react-router';
-import { ExternalLink, RefreshCw, Pencil, LayoutGrid, List, ChevronRight, ChevronDown, ArrowUp, ArrowDown } from 'lucide-react';
+import { ExternalLink, RefreshCw, Pencil, LayoutGrid, List, ChevronRight, ChevronDown, ArrowUp, ArrowDown, Search } from 'lucide-react';
 import {
   StatCard, StatusDot, Badge, MonoText,
   GroupCard, DataTable, EventFeed,
@@ -708,10 +708,11 @@ export default function AgentHealth() {
             </button>
           </div>
           <div className={styles.appFilterWrap}>
+            <Search size={12} className={styles.appFilterIcon} />
             <input
               type="text"
               className={styles.appFilterInput}
-              placeholder="Filter apps..."
+              placeholder="Filter..."
               value={appFilter}
               onChange={(e) => setAppFilter(e.target.value)}
               aria-label="Filter applications"