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>

backend/app/main.py

@@ -5,7 +5,7 @@ from fastapi import Depends, FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.api import auth as auth_api
-from app.api import chat, logs, memory, notes, system
+from app.api import chat, knowledge_base, logs, memory, notes, system
 from app.api import models as models_api
 from app.api import settings as settings_api
 from app.core import settings_store
@@ -94,6 +94,7 @@ app.include_router(auth_api.router)
 app.include_router(chat.router, dependencies=[Depends(require_site_session)])
 app.include_router(memory.router, dependencies=[Depends(require_site_session)])
 app.include_router(notes.router, dependencies=[Depends(require_site_session)])
+app.include_router(knowledge_base.router, dependencies=[Depends(require_site_session)])
 app.include_router(logs.router, dependencies=[Depends(require_site_session)])
 app.include_router(system.router, dependencies=[Depends(require_site_session)])
 app.include_router(settings_api.router, dependencies=[Depends(require_site_session)])