feat(frontend): message search and session export entry points

- SearchModal queries GET /messages/search and jumps to the matching
  session; opened from the sidebar button or Ctrl+K
- session items in the sidebar gain a hover export button that
  downloads the markdown export via an authenticated fetch + blob
  (plain links cannot send the JWT)

frontend/web/src/components/chat/SearchModal.tsx

@@ -0,0 +1,86 @@
+import { useState } from 'react'
+import { useNavigate } from 'react-router-dom'
+import { api } from '../../api/client'
+
+interface SearchResult {
+  id: number
+  session_id: string
+  role: string
+  content: string
+  created_at: string
+}
+
+function snippet(content: string, q: string): string {
+  const idx = content.indexOf(q)
+  if (idx < 0) return content.slice(0, 80)
+  const start = Math.max(0, idx - 20)
+  return (start > 0 ? '…' : '') + content.slice(start, start + 80) + (start + 80 < content.length ? '…' : '')
+}
+
+export default function SearchModal({ onClose }: { onClose: () => void }) {
+  const navigate = useNavigate()
+  const [q, setQ] = useState('')
+  const [results, setResults] = useState<SearchResult[]>([])
+  const [searched, setSearched] = useState(false)
+  const [loading, setLoading] = useState(false)
+
+  const search = async () => {
+    const query = q.trim()
+    if (!query) return
+    setLoading(true)
+    try {
+      const data = await api.messages.search(query)
+      setResults(data.messages || [])
+      setSearched(true)
+    } catch { /* ignore */ } finally {
+      setLoading(false)
+    }
+  }
+
+  const jump = (sessionId: string) => {
+    onClose()
+    navigate(`/chat/${sessionId}`)
+  }
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-start justify-center pt-24" style={{ background: 'rgba(0,0,0,0.4)' }}
+      onClick={onClose}>
+      <div className="w-full max-w-lg rounded-[18px] border p-4 space-y-3"
+        style={{ background: 'var(--mx-surface)', borderColor: 'var(--mx-outline)' }}
+        onClick={(e) => e.stopPropagation()}>
+        <div className="flex gap-2">
+          <input
+            autoFocus
+            value={q}
+            onChange={(e) => setQ(e.target.value)}
+            onKeyDown={(e) => e.key === 'Enter' && search()}
+            placeholder="搜索所有会话的消息…"
+            className="flex-1 rounded-[14px] border px-3 py-2 text-sm outline-none"
+            style={{ background: 'var(--mx-bg)', borderColor: 'var(--mx-outline)', color: 'var(--mx-text)' }}
+          />
+          <button onClick={search} disabled={loading}
+            className="px-4 py-2 rounded-full text-sm font-medium text-white"
+            style={{ background: 'var(--mx-primary)' }}>
+            {loading ? '…' : '搜索'}
+          </button>
+        </div>
+
+        <div className="max-h-80 overflow-auto space-y-1">
+          {searched && results.length === 0 && (
+            <p className="text-center text-sm py-6" style={{ color: 'var(--mx-text-dim)' }}>没有找到相关消息</p>
+          )}
+          {results.map((m) => (
+            <div key={m.id} onClick={() => jump(m.session_id)}
+              className="px-3 py-2 rounded-[14px] cursor-pointer transition-colors hover:opacity-80"
+              style={{ background: 'var(--mx-bg)' }}>
+              <p className="text-sm" style={{ color: 'var(--mx-text)' }}>{snippet(m.content, q)}</p>
+              <p className="text-xs mt-0.5" style={{ color: 'var(--mx-text-dim)' }}>
+                {m.role === 'user' ? '你' : '昔涟'} · {new Date(m.created_at).toLocaleString('zh-CN')}
+              </p>
+            </div>
+          ))}
+        </div>
+      </div>
+    </div>
+  )
+}