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>

frontend/src/components/SidePanelMenu.vue

@@ -0,0 +1,118 @@
+<script setup lang="ts">
+import { computed, onMounted, onUnmounted, ref } from 'vue'
+import { SIDE_PANELS, type PanelId } from '../panels'
+
+const props = defineProps<{ active: PanelId | null }>()
+const emit = defineEmits<{ (e: 'select', panel: PanelId | null): void }>()
+
+const open = ref(false)
+const root = ref<HTMLElement | null>(null)
+
+const activePanel = computed(() => SIDE_PANELS.find((p) => p.id === props.active) ?? null)
+
+/** 項目を選ぶ。開いているパネルをもう一度選んだら閉じる（従来のトグルと同じ操作感）。 */
+function select(id: PanelId) {
+  emit('select', props.active === id ? null : id)
+  open.value = false
+}
+
+function close() {
+  emit('select', null)
+  open.value = false
+}
+
+function onDocumentPointerDown(e: PointerEvent) {
+  if (open.value && !root.value?.contains(e.target as Node)) open.value = false
+}
+
+function onKeydown(e: KeyboardEvent) {
+  if (e.key === 'Escape') open.value = false
+}
+
+onMounted(() => {
+  document.addEventListener('pointerdown', onDocumentPointerDown)
+  document.addEventListener('keydown', onKeydown)
+})
+onUnmounted(() => {
+  document.removeEventListener('pointerdown', onDocumentPointerDown)
+  document.removeEventListener('keydown', onKeydown)
+})
+</script>
+
+<template>
+  <div ref="root" class="panel-menu">
+    <button
+      class="menu-btn"
+      :class="{ active: activePanel !== null, open }"
+      :title="activePanel ? `${activePanel.label}を表示中` : 'パネルを開く'"
+      @click="open = !open"
+    >
+      <span class="menu-icon">{{ activePanel ? activePanel.icon : '☰' }}</span>
+      <span class="menu-label">{{ activePanel ? activePanel.label : 'パネル' }}</span>
+      <span class="caret">▾</span>
+    </button>
+
+    <div v-if="open" class="dropdown">
+      <button
+        v-for="panel in SIDE_PANELS"
+        :key="panel.id"
+        class="item"
+        :class="{ current: panel.id === active }"
+        @click="select(panel.id)"
+      >
+        <span class="item-icon">{{ panel.icon }}</span>
+        <span class="item-text">
+          <span class="item-label">{{ panel.label }}</span>
+          <span class="item-desc">{{ panel.description }}</span>
+        </span>
+        <span class="check">{{ panel.id === active ? '✓' : '' }}</span>
+      </button>
+      <button v-if="activePanel" class="item close-item" @click="close">
+        <span class="item-icon">✕</span>
+        <span class="item-text"><span class="item-label">パネルを閉じる</span></span>
+        <span class="check" />
+      </button>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.panel-menu { position: relative; }
+.menu-btn {
+  display: flex; align-items: center; gap: 6px;
+  background: #2a2a2a; color: #aaa; border: 1px solid #444;
+  border-radius: 6px; padding: 4px 12px; font-size: 13px; cursor: pointer;
+}
+.menu-btn:hover { color: #ddd; }
+.menu-btn.active { background: #1d3557; color: #93c5fd; border-color: #2563eb; }
+.menu-btn.open { border-color: #666; }
+.menu-btn.active.open { border-color: #3b82f6; }
+.menu-icon { font-size: 13px; line-height: 1; }
+.menu-label { white-space: nowrap; }
+.caret { font-size: 9px; color: #777; }
+
+.dropdown {
+  position: absolute; top: calc(100% + 6px); right: 0; z-index: 50;
+  min-width: 260px;
+  background: #1c1c1c; border: 1px solid #3a3a3a; border-radius: 8px;
+  padding: 4px; box-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
+}
+.item {
+  display: flex; align-items: center; gap: 10px; width: 100%;
+  background: none; border: none; border-radius: 6px;
+  padding: 7px 9px; cursor: pointer; text-align: left;
+}
+.item:hover { background: #262626; }
+.item.current { background: #1d3557; }
+.item-icon { font-size: 14px; width: 18px; text-align: center; flex-shrink: 0; }
+.item-text { flex: 1; display: flex; flex-direction: column; gap: 1px; overflow: hidden; }
+.item-label { color: #ddd; font-size: 13px; }
+.item.current .item-label { color: #93c5fd; }
+.item-desc {
+  color: #6b7280; font-size: 11px;
+  overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
+}
+.check { color: #93c5fd; font-size: 11px; width: 10px; flex-shrink: 0; }
+.close-item { border-top: 1px solid #2a2a2a; border-radius: 0 0 6px 6px; margin-top: 2px; }
+.close-item .item-label { color: #888; }
+</style>