Add session export/import as JSON

Sessions can now be downloaded as a JSON file containing the session
metadata and every message with its tags, and re-imported as a new
session from that same format.

GET /chat/sessions/{id}/export streams the session as a file download.
POST /chat/sessions/import validates the roles, then recreates the
session under a fresh UUID so importing never collides with an existing
session. The sidebar gets a per-row download button and an import button
that feeds a hidden file input.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/stores/chat.ts

@@ -1,6 +1,6 @@
 import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
-import { fetchSessions, fetchMessages, deleteSession, streamChat, type SessionInfo, type Source } from '../api/client'
+import { fetchSessions, fetchMessages, deleteSession, exportSession, importSession, streamChat, type SessionInfo, type Source, type ImportPayload } from '../api/client'
 
 export interface Message {
   role: 'user' | 'assistant'
@@ -69,6 +69,32 @@ export const useChatStore = defineStore('chat', () => {
     await loadSessions()
   }
 
+  function downloadSessionLog(sessionId: string): void {
+    exportSession(sessionId)
+  }
+
+  async function importSessionFromFile(file: File): Promise<void> {
+    let payload: ImportPayload
+    try {
+      const parsed = JSON.parse(await file.text())
+      if (!Array.isArray(parsed?.messages)) {
+        throw new Error('messages 配列が見つかりません')
+      }
+      payload = parsed as ImportPayload
+    } catch (e) {
+      pushToast(`インポート失敗: JSONの読み込みに失敗しました (${e instanceof Error ? e.message : e})`)
+      return
+    }
+    try {
+      const session = await importSession(payload)
+      await loadSessions()
+      await selectSession(session.id)
+      pushToast(`「${session.title}」をインポートしました`)
+    } catch (e) {
+      pushToast(`インポート失敗: ${e instanceof Error ? e.message : e}`)
+    }
+  }
+
   function sendMessage(text: string): void {
     if (isStreaming.value || !text.trim()) return
 
@@ -137,6 +163,8 @@ export const useChatStore = defineStore('chat', () => {
     loadSessions,
     selectSession,
     removeSession,
+    downloadSessionLog,
+    importSessionFromFile,
     sendMessage,
     retryMessage,
   }