chore(release): 2.4.0

- 版本号 2.3.4 → 2.4.0（tauri.conf.json / README）
- CHANGELOG 补充 2.4.0、回填 2.3.4

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

BillNote_frontend/src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "../node_modules/@tauri-apps/cli/config.schema.json",
   "productName": "BiliNote",
-  "version": "2.3.4",
+  "version": "2.4.0",
   "identifier": "com.jefferyhuang.bilinote",
   "build": {
     "frontendDist": "../dist",