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>

src/db.rs

@@ -101,6 +101,35 @@ impl Database {
         .execute(&self.pool)
         .await;
 
+        // KB tables
+        sqlx::query(
+            "CREATE TABLE IF NOT EXISTS kb_content (
+                id INTEGER PRIMARY KEY CHECK (id = 1),
+                content TEXT NOT NULL DEFAULT '',
+                updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+            )"
+        )
+        .execute(&self.pool)
+        .await?;
+
+        // Insert default row if not exists
+        let _ = sqlx::query(
+            "INSERT OR IGNORE INTO kb_content (id, content) VALUES (1, '')"
+        )
+        .execute(&self.pool)
+        .await;
+
+        sqlx::query(
+            "CREATE TABLE IF NOT EXISTS kb_chunks (
+                id TEXT PRIMARY KEY,
+                title TEXT NOT NULL DEFAULT '',
+                content TEXT NOT NULL,
+                embedding BLOB NOT NULL
+            )"
+        )
+        .execute(&self.pool)
+        .await?;
+
         sqlx::query(
             "CREATE TABLE IF NOT EXISTS timers (
                 id TEXT PRIMARY KEY,