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/pages/chat/ChatPage.tsx

@@ -1,7 +1,8 @@
-import { useEffect } from 'react'
+import { useEffect, useState } from 'react'
 import { useParams } from 'react-router-dom'
 import { useChatStore } from '../../store/chatStore'
 import ChatContainer from '../../components/chat/ChatContainer'
+import SearchModal from '../../components/chat/SearchModal'
 import Sidebar from '../../components/layout/Sidebar'
 import { api } from '../../api/client'
 import type { Message } from '../../types/chat'
@@ -9,6 +10,7 @@ import type { Message } from '../../types/chat'
 export default function ChatPage() {
   const { sessionId } = useParams()
   const { setSessionId, setMessages } = useChatStore()
+  const [showSearch, setShowSearch] = useState(false)
 
   useEffect(() => {
     if (!sessionId) return
@@ -30,12 +32,26 @@ export default function ChatPage() {
     return () => { cancelled = true }
   }, [sessionId, setSessionId, setMessages])
 
+  // Ctrl+K opens message search
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      if ((e.ctrlKey || e.metaKey) && e.key === 'k') {
+        e.preventDefault()
+        setShowSearch(true)
+      }
+    }
+    window.addEventListener('keydown', onKey)
+    return () => window.removeEventListener('keydown', onKey)
+  }, [])
+
   return (
     <div className="flex h-full">
-      <Sidebar />
+      <Sidebar onSearch={() => setShowSearch(true)} />
       <div className="flex-1 min-w-0">
         <ChatContainer />
       </div>
+      {showSearch && <SearchModal onClose={() => setShowSearch(false)} />}
     </div>
   )
 }
+