Add global knowledge base with RAG search

- KB module: fastembed (AllMiniLML6V2) for CPU embedding, SQLite for
  vector storage with brute-force cosine similarity search
- Chunking by ## headings, embeddings stored as BLOB in kb_chunks table
- API: GET/PUT /api/kb for full-text read/write with auto re-indexing
- Agent tools: kb_search (top-5 semantic search) and kb_read (full text)
  available in both planning and execution phases
- Frontend: Settings menu in sidebar footer, KB editor as independent
  view with markdown textarea and save button
- Also: extract shared db_err/ApiResult to api/mod.rs, add context
  management design doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/components/Sidebar.vue

@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { ref } from 'vue'
 import type { Project } from '../types'
 
 defineProps<{
@@ -10,14 +11,22 @@ const emit = defineEmits<{
   select: [id: string]
   create: []
   delete: [id: string]
+  openKb: []
 }>()
 
+const showSettings = ref(false)
+
 function onDelete(e: Event, id: string) {
   e.stopPropagation()
   if (confirm('确定删除这个项目？')) {
     emit('delete', id)
   }
 }
+
+function onOpenKb() {
+  showSettings.value = false
+  emit('openKb')
+}
 </script>
 
 <template>
@@ -41,6 +50,14 @@ function onDelete(e: Event, id: string) {
         <span class="project-time">{{ new Date(project.updated_at).toLocaleDateString() }}</span>
       </div>
     </nav>
+    <div class="sidebar-footer">
+      <div class="settings-wrapper">
+        <button class="btn-settings" @click="showSettings = !showSettings">Settings</button>
+        <div v-if="showSettings" class="settings-menu">
+          <button class="settings-item" @click="onOpenKb">Knowledge Base</button>
+        </div>
+      </div>
+    </div>
   </aside>
 </template>
 
@@ -153,4 +170,59 @@ function onDelete(e: Event, id: string) {
   font-size: 11px;
   color: var(--text-secondary);
 }
+
+.sidebar-footer {
+  padding: 12px 16px;
+  border-top: 1px solid var(--border);
+}
+
+.settings-wrapper {
+  position: relative;
+}
+
+.btn-settings {
+  width: 100%;
+  padding: 8px;
+  background: transparent;
+  color: var(--text-secondary);
+  border: none;
+  font-size: 13px;
+  cursor: pointer;
+  text-align: left;
+  border-radius: 6px;
+}
+
+.btn-settings:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.settings-menu {
+  position: absolute;
+  bottom: 100%;
+  left: 0;
+  right: 0;
+  margin-bottom: 4px;
+  background: var(--bg-primary);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 4px;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
+}
+
+.settings-item {
+  width: 100%;
+  padding: 8px 12px;
+  background: none;
+  border: none;
+  color: var(--text-primary);
+  font-size: 13px;
+  text-align: left;
+  cursor: pointer;
+  border-radius: 6px;
+}
+
+.settings-item:hover {
+  background: var(--bg-tertiary);
+}
 </style>