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/components/SessionList.vue

@@ -4,6 +4,7 @@ import { useChatStore } from '../stores/chat'
 
 const store = useChatStore()
 const error = ref<string | null>(null)
+const importInput = ref<HTMLInputElement | null>(null)
 
 async function load() {
   error.value = null
@@ -14,6 +15,18 @@ async function load() {
   }
 }
 
+function triggerImport() {
+  importInput.value?.click()
+}
+
+async function onImportFileChange(e: Event) {
+  const input = e.target as HTMLInputElement
+  const file = input.files?.[0]
+  input.value = ''
+  if (!file) return
+  await store.importSessionFromFile(file)
+}
+
 onMounted(load)
 </script>
 
@@ -21,7 +34,17 @@ onMounted(load)
   <aside class="session-list">
     <div class="header">
       <span>チャット履歴</span>
-      <button class="new-btn" @click="store.selectSession(null)">+ 新規</button>
+      <div class="header-actions">
+        <button class="import-btn" title="JSONからインポート" @click="triggerImport">⭱</button>
+        <button class="new-btn" @click="store.selectSession(null)">+ 新規</button>
+      </div>
+      <input
+        ref="importInput"
+        type="file"
+        accept="application/json,.json"
+        class="hidden-file-input"
+        @change="onImportFileChange"
+      >
     </div>
     <div v-if="error" class="api-error" :title="error">⚠ 接続エラー</div>
     <ul>
@@ -32,6 +55,11 @@ onMounted(load)
         @click="store.selectSession(s.id)"
       >
         <span class="title">{{ s.title }}</span>
+        <button
+          class="export-btn"
+          @click.stop="store.downloadSessionLog(s.id)"
+          title="JSONでエクスポート"
+        >⭳</button>
         <button
           class="del-btn"
           @click.stop="store.removeSession(s.id)"
@@ -69,7 +97,13 @@ onMounted(load)
   border-bottom: 1px solid #7f1d1d;
   cursor: default;
 }
-.new-btn {
+.header-actions {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+.new-btn,
+.import-btn {
   background: #2a2a2a;
   color: #ccc;
   border: 1px solid #444;
@@ -78,6 +112,9 @@ onMounted(load)
   font-size: 12px;
   cursor: pointer;
 }
+.hidden-file-input {
+  display: none;
+}
 ul {
   list-style: none;
   margin: 0;
@@ -104,6 +141,7 @@ li.active { background: #333; }
   font-size: 13px;
   color: #ddd;
 }
+.export-btn,
 .del-btn {
   background: none;
   border: none;
@@ -114,5 +152,7 @@ li.active { background: #333; }
   opacity: 0;
   transition: opacity 0.15s;
 }
+.export-btn { font-size: 13px; }
+li:hover .export-btn,
 li:hover .del-btn { opacity: 1; }
 </style>