Browse the knowledge base from a single side panel menu

Add a read-only view of backend/knowledge_base/**.md so the files that get
injected into the system prompt can be inspected from the GUI:

- GET /api/kb/files lists the Markdown files (title, preview, size, mtime and
  whether the file is already embedded), GET /api/kb/file returns one in full.
  Paths are resolved against the knowledge base directory and rejected unless
  they stay inside it and end in .md.
- KnowledgeBasePanel.vue groups the files by folder (collapsible, so web_search
  can be folded away), filters by title/path/preview and renders the selected
  file with MarkdownContent.

The header had grown to four side panel toggles, so collapse them into one
menu: panels.ts now holds the list of panels along with the component each one
renders, App.vue picks it with <component :is>, and SidePanelMenu.vue shows the
active panel on its button. Re-picking the open panel (or "パネルを閉じる")
closes it; a click outside or Esc closes just the menu.

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

docs/architecture.md

@@ -26,6 +26,7 @@ backend/
 │   │   ├── chat.py          # チャット・セッション関連エンドポイント（SSE / export / import）
 │   │   ├── memory.py        # 長期メモリ参照・削除
 │   │   ├── notes.py         # 共有ノート CRUD
+│   │   ├── knowledge_base.py # ローカル KB の Markdown を GUI から閲覧（読み取り専用）
 │   │   ├── models.py        # モデルプリセット CRUD
 │   │   ├── settings.py      # GUI 設定画面（APIキー等）
 │   │   ├── system.py        # タグ backfill・ワードプール参照などの運用系